/in/foo.c: In function 'main':
/in/foo.c:6:2: error: 'flat' undeclared (first use in this function)
  flat=1;
  ^~~~
/in/foo.c:6:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:4:10: warning: unused variable 'flag' [-Wunused-variable]
  int n,i,flag;
          ^~~~