Fixed subtraction

tree.go

@@ -259,10 +259,21 @@ func (operation *Operation) Solution () (solution int64, err error) {
 			solution += subSolution
 		}
 	case OpcodeSubtract:
-		for _, operand := range operation.operands {
+		if len(operation.operands) == 1 {
+			solution, err = operation.operands[0].Solution()
+			solution *= -1
+			if err != nil { return }
+			break
+		}
+	
+		for index, operand := range operation.operands {
 			subSolution, err = operand.Solution()
 			if err != nil { return }
-			solution -= subSolution
+			if index == 0 {
+				solution = subSolution
+			} else {
+				solution -= subSolution
+			}
 		}
 	case OpcodeMultiply:
 		solution = 1