foo.cc: In function 'int main()':
foo.cc:9:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  {scanf("%d",&a[i]); sum+=a[i];}
   ~~~~~^~~~~~~~~~~~
foo.cc:9:25: warning: 'sum' may be used uninitialized in this function [-Wmaybe-uninitialized]
  {scanf("%d",&a[i]); sum+=a[i];}
                      ~~~^~~~~~

[Hydro](https://hydro.ac)提供评测服务