/in/foo.c: In function 'main':
/in/foo.c:6:10: warning: format '%d' expects argument of type 'int *', but argument 2 has type 'int' [-Wformat=]
  scanf("%d",a);
          ^
/in/foo.c:7:10: warning: format '%d' expects argument of type 'int *', but argument 2 has type 'int' [-Wformat=]
  scanf("%d",b);
          ^
/in/foo.c:8:2: error: 'sum' undeclared (first use in this function)
  sum=a+b;
  ^~~
/in/foo.c:8:2: note: each undeclared identifier is reported only once for each function it appears in