diff --git a/src/main/java/com/ordestiny/tdd/kata/StringCalculator.java b/src/main/java/com/ordestiny/tdd/kata/StringCalculator.java
new file mode 100755
index 0000000..3bab77e
--- /dev/null
+++ b/src/main/java/com/ordestiny/tdd/kata/StringCalculator.java
@@ -0,0 +1,92 @@
+package com.ordestiny.tdd.kata;
+
+import java.text.DecimalFormat;
+import java.util.Arrays;
+import java.util.regex.Pattern;
+
+public class StringCalculator {
+
+	private static final String NEWLINE = "\n";
+	private static final String CUSTOM_DELIMITER_SUFFIX = NEWLINE;
+	private static final String DEFAULT_DELIMITERS = ",|" + NEWLINE;
+	private static final String CUSTOM_DELIMITER_PREFIX = "//";
+
+	public String add(String number) {
+		String delimiter = DEFAULT_DELIMITERS;
+		String numbers = number;
+
+		if (number.startsWith(CUSTOM_DELIMITER_PREFIX)) {
+			delimiter = getCustomDelimiter(number);
+			numbers = getNumbersInput(number);
+			validateInput(numbers, delimiter);
+		}
+
+		if (numbers.isEmpty()) {
+			return "0";
+		}
+
+		if (isInvalidLastCharacterIn(numbers)) {
+			throw new IllegalArgumentException("Number expected but found EOF");
+		}
+
+		double result = getSum(numbers, delimiter);
+		return format(result);
+	}
+
+	private String getCustomDelimiter(String input) {
+		int indexOfDelimiterStart = input.lastIndexOf(CUSTOM_DELIMITER_PREFIX) + 2;
+		int indexOfNumbersStart = input.indexOf(CUSTOM_DELIMITER_SUFFIX);
+
+		String unquotedDelimiter = input.substring(indexOfDelimiterStart, indexOfNumbersStart);
+
+		return unquotedDelimiter;
+	}
+
+	private String getNumbersInput(String input) {
+		return input.substring(input.lastIndexOf(NEWLINE) + 1);
+	}
+
+	private boolean isInvalidLastCharacterIn(String numbers) {
+		return Character.digit(numbers.charAt(numbers.length() - 1), 10) < 0;
+	}
+
+	private void validateInput(String input, String delimiter) {
+		for (int i = 1; i < input.length(); i += 2) {
+			int expectedDelimiterIndex = i + delimiter.length();
+
+			if (expectedDelimiterIndex < input.length()) {
+				String delimiterInString = input.substring(i, expectedDelimiterIndex);
+
+				if (!delimiterInString.equals(delimiter)) {
+					String exceptionMessage = String.format("'%s' expected but '%s', found at position %d", delimiter,
+							delimiterInString, i);
+					throw new IllegalArgumentException(exceptionMessage);
+				}
+			}
+		}
+	}
+
+	private double getSum(String numbers, String delimiter) {
+		if (delimiter.matches("[^A-Za-z0-9]")) {
+			// Quote any delimiter that is a special character, such as `|`
+			delimiter = Pattern.quote(delimiter);
+		}
+
+		String[] arrayOfStringNumbers = numbers.split(delimiter);
+
+		return sum(arrayOfStringNumbers);
+	}
+
+	private double sum(String[] numbers) {
+		Double sum = Arrays.stream(numbers)
+				.map(number-> Double.valueOf(number))
+				.reduce(0.0, Double::sum);
+		
+		return sum;
+	}
+
+	private String format(double value) {
+		DecimalFormat format = new DecimalFormat("0.#");
+		return format.format(value);
+	}
+}
diff --git a/src/test/java/com/ordestiny/tdd/kata/StringCalculatorTest.java b/src/test/java/com/ordestiny/tdd/kata/StringCalculatorTest.java
new file mode 100755
index 0000000..0e36e35
--- /dev/null
+++ b/src/test/java/com/ordestiny/tdd/kata/StringCalculatorTest.java
@@ -0,0 +1,41 @@
+package com.ordestiny.tdd.kata;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+import org.junit.jupiter.params.provider.ValueSource;
+
+public class StringCalculatorTest {
+
+	private static StringCalculator fixture;
+
+	@BeforeAll
+	public static void setup() {
+		fixture = new StringCalculator();
+	}
+
+	@ParameterizedTest
+	@CsvSource({ "'',0", "1,1", "'1.1,2.2',3.3", "'1\n2,3', 6", "'//;\n1;2',3", "'//|\n1|2|3', 6", "'//sep\n2sep3',5",
+			"'//;\n',0" })
+	public void add_WithValidNumber_SumOfNumbers(String input, String expected) {
+		final String actual = fixture.add(input);
+
+		assertEquals(expected, actual);
+	}
+
+	@ParameterizedTest
+	@ValueSource(strings = { "//|\n1|2,3", "//a\n1a2a3a4b5", "//%\n1%4$5", "//c\n1c2c" })
+	public void add_WithInvalidInput_ThrowException(String input) {
+		Exception exception = assertThrows(IllegalArgumentException.class, () -> {
+			fixture.add(input);
+		});
+
+		String expectedMessage = "expected but";
+		String actualMessage = exception.getMessage();
+		assertTrue(actualMessage.contains(expectedMessage));
+	}
+}