/in/foo.c: In function 'main':
/in/foo.c:6:15: warning: implicit declaration of function 'malloc' [-Wimplicit-function-declaration]
  int*a =(int*)malloc(n*sizeof(int));
               ^~~~~~
/in/foo.c:6:15: warning: incompatible implicit declaration of built-in function 'malloc'
/in/foo.c:6:15: note: include '<stdlib.h>' or provide a declaration of 'malloc'
/in/foo.c:12:2: warning: implicit declaration of function 'system' [-Wimplicit-function-declaration]
  system("pause");
  ^~~~~~
/in/foo.c:4:10: warning: unused variable 'sum' [-Wunused-variable]
  int n,i,sum = 2;
          ^~~