diff --git a/src/main/kotlin/CustomAdt.kt b/src/main/kotlin/CustomAdt.kt
index c30e0c4..5c09f41 100644
--- a/src/main/kotlin/CustomAdt.kt
+++ b/src/main/kotlin/CustomAdt.kt
@@ -8,8 +8,20 @@ sealed interface ParseResult {
 
 object CustomAdt {
 
-    fun parseInt(str: String): ParseResult {
-        TODO()
+    fun parseInt(input: String): ParseResult {
+        val (sign, elementsToSkip) = if (input.startsWith("-")) -1 to 1 else 1 to 0
+
+        return input.withIndex()
+            .drop(elementsToSkip)
+            .map { (index, char) ->
+                if (char in '0'..'9') {
+                    char - '0'
+                } else {
+                    return ParseResult.Failure(index, char)
+                }
+            }
+            .fold(0) { acc, digit -> acc * 10 + digit }
+            .let { result -> ParseResult.Success(result * sign) }
     }
 
 }
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