View Compilation Error

0_0_28023506_19963.cpp: In function 'int main()':
0_0_28023506_19963.cpp:11:30: error: 'n' was not declared in this scope
  while(scanf("%d%d",&a,&b)&&(n+m)){
                              ^
0_0_28023506_19963.cpp:11:32: error: 'm' was not declared in this scope
  while(scanf("%d%d",&a,&b)&&(n+m)){
                                ^
0_0_28023506_19963.cpp:12:18: error: '_gcd' was not declared in this scope
   int g=-_gcd(a,b);
                  ^