foo.c: In function 'Sum':
foo.c:3:5: warning: type of 'x' defaults to 'int' [-Wimplicit-int]
 int Sum(x)       //定义Sum函数
     ^~~
foo.c: In function 'main':
foo.c:19:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&a);
  ^~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务