I'm having a problem with gcd. I don't know why it gives me a red squiggly under the gcd(lower case) at this line

GCD = gcd(y, x % y);

Any help?

Code:// Greatest Common Divisor.cpp : Defines the entry point for the console application. // // ========= // Libraries // ================== #include "stdafx.h" #include <iostream> #include <math.h> #include <stdio.h> // ================== // ========== // Namespaces // ==================== using namespace std; // ==================== // =================== // Function Prototypes // ===================== void WelcomeBanner(); void GetIntegers(int&, int&); // ===================== // ============== int main( ) { // main() int x; int y; int GCD; WelcomeBanner(); GetIntegers(x, y); if (y == 0 ){ cout << "GCD = " << x << endl; } else GCD = gcd(y, x % y); cout << "GCD = " << GCD << endl; return 0; }// main() // ======== // ========================= // Function WelcomeBanner() // ========================= void WelcomeBanner(){ cout << "================================================" << endl; cout << "Welcome to my program that will give you the |" << endl << "greatest common divisor of your two numbers =) |" << endl; cout << "================================================"<< endl << endl; }// Welcome banner // ================== // ==================== // Function GetIntegers // ==================== void GetIntegers(int& num1, int& num2){ cout << "Please enter an integer that you want me to find the GCD of." << endl; cin >> num1; cout << "Please enter another integer that you want me to find the GCD of." << endl; cin >> num2; }//GetIntegers // ==============