foo.cpp: In function 'int main(int, char**)':
foo.cpp:19:19: error: 'system' was not declared in this scope
     system("pause");
                   ^
foo.cpp:13:8: warning: unused variable 'b' [-Wunused-variable]
  int a,b,c,d,e,f;
        ^
foo.cpp:13:10: warning: unused variable 'c' [-Wunused-variable]
  int a,b,c,d,e,f;
          ^
foo.cpp:13:12: warning: unused variable 'd' [-Wunused-variable]
  int a,b,c,d,e,f;
            ^
foo.cpp:13:14: warning: unused variable 'e' [-Wunused-variable]
  int a,b,c,d,e,f;
              ^
foo.cpp:13:16: warning: unused variable 'f' [-Wunused-variable]
  int a,b,c,d,e,f;
                ^