View Compilation Error

0_0_18161571_6977.cpp: In function 'int main()':
0_0_18161571_6977.cpp:22:28: error: '__gcd' was not declared in this scope
                if(__gcd(m,n)== i)
                            ^
0_0_18161571_6977.cpp:30:29: error: '__gcd' was not declared in this scope
                 if(__gcd(n,m) == k2)
                             ^