View Compilation Error

0_0_36986068_16685.cpp: In function 'int main()':
0_0_36986068_16685.cpp:25:15: error: '__gcd' was not declared in this scope
    a=__gcd(a,b);
               ^
0_0_36986068_16685.cpp:25:15: note: suggested alternative:
In file included from GCC4.9.2/x86_64-w64-mingw32/include/c++/algorithm:62:0,
                 from GCC4.9.2/x86_64-w64-mingw32/include/c++/x86_64-w64-mingw32/bits/stdc++.h:64,
                 from 0_0_36986068_16685.cpp:1:
GCC4.9.2/x86_64-w64-mingw32/include/c++/bits/stl_algo.h:1229:5: note:   'std::__gcd'
     __gcd(_EuclideanRingElement __m, _EuclideanRingElement __n)
     ^