/in/foo.cc: In function 'int main()':
/in/foo.cc:7:8: error: 'F' was not declared in this scope
   cin>>F;
        ^
/in/foo.cc:6:9: warning: unused variable 'a' [-Wunused-variable]
   float a,b;
         ^
/in/foo.cc:6:11: warning: unused variable 'b' [-Wunused-variable]
   float a,b;
           ^