View Compilation Error

0_0_38500851_24399.cpp: In function 'll Pollard_Rho(ll)':
0_0_38500851_24399.cpp:86:31: error: 'gcd' was not declared in this scope
                 ll d=gcd(val,x);
                               ^
0_0_38500851_24399.cpp:90:23: error: 'gcd' was not declared in this scope
         ll d=gcd(val,x);
                       ^