foo.cc: In function ‘int main()’:
foo.cc:13:22: warning: ‘n’ is used uninitialized [-Wuninitialized]
   13 |         for(int i=1;i<=n;i++)
      |                     ~^~~
foo.cc:11:11: note: ‘n’ was declared here
   11 |     int a,n;
      |           ^