/in/foo.c: In function 'main':
/in/foo.c:6:2: error: 'sum' undeclared (first use in this function)
  sum=a+b;
  ^~~
/in/foo.c:6:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:5:10: warning: unused variable 'c' [-Wunused-variable]
  int a,b,c;
          ^