foo.cc: In function 'int main()':
foo.cc:5:6: warning: unused variable 'a' [-Wunused-variable]
  int a,b,m,n;
      ^
foo.cc:5:8: warning: unused variable 'b' [-Wunused-variable]
  int a,b,m,n;
        ^
foo.cc:5:10: warning: unused variable 'm' [-Wunused-variable]
  int a,b,m,n;
          ^
foo.cc:5:12: warning: unused variable 'n' [-Wunused-variable]
  int a,b,m,n;
            ^