/in/foo.c: In function 'main':
/in/foo.c:6:17: warning: implicit declaration of function 'malloc' [-Wimplicit-function-declaration]
  int *a = (int*)malloc(n * sizeof(int));
                 ^~~~~~
/in/foo.c:6:17: warning: incompatible implicit declaration of built-in function 'malloc'
/in/foo.c:6:17: note: include '<stdlib.h>' or provide a declaration of 'malloc'
/in/foo.c:14:2: warning: implicit declaration of function 'system' [-Wimplicit-function-declaration]
  system("pause");
  ^~~~~~
/in/foo.c:4:12: warning: unused variable 'sum' [-Wunused-variable]
  int n, i, sum = 2;
            ^~~