foo.cc: In function 'int main()':
foo.cc:11:17: warning: variable 'b' set but not used [-Wunused-but-set-variable]
     int n,ans=0,b=0;
                 ^
foo.cc:12:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&n);
     ~~~~~^~~~~~~~~
foo.cc:14:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&a[i]);
     ~~~~~^~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务