/in/foo.c: In function 'main':
/in/foo.c:6:2: error: 'cin' undeclared (first use in this function)
  cin >>a,b;
  ^~~
/in/foo.c:6:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:6:9: warning: left-hand operand of comma expression has no effect [-Wunused-value]
  cin >>a,b;
         ^
/in/foo.c:6:2: warning: statement with no effect [-Wunused-value]
  cin >>a,b;
  ^~~
/in/foo.c:7:5: error: 'cout' undeclared (first use in this function)
     cout <<a+b;
     ^~~~