/in/foo.c: In function 'main':
/in/foo.c:5:12: warning: unused variable 'p' [-Wunused-variable]
  int m, n, p;
            ^
/in/foo.c:7:2: warning: 'n' is used uninitialized in this function [-Wuninitialized]
  scanf("%d", &a[n]);
  ^~~~~~~~~~~~~~~~~~