foo.cpp: In function 'int ext_gcd(int, int, int&, int&)':
foo.cpp:27:2: error: 'ans' was not declared in this scope
  ans=ext_gcd(b,a%b,y,x);
  ^
foo.cpp: In function 'int main()':
foo.cpp:33:6: warning: unused variable 'gcd' [-Wunused-variable]
  int gcd=ext_gcd(a,b,x,y);
      ^
foo.cpp: In function 'int ext_gcd(int, int, int&, int&)':
foo.cpp:30:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^