foo.cc: In function 'int main()':
foo.cc:3:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 #define rep(i,l,r) for(int i=l;i<=r;++i)
foo.cc:19:13:
         rep(i,1,s.size())
             ~~~~~~~~~~~~         
foo.cc:19:9: note: in expansion of macro 'rep'
         rep(i,1,s.size())
         ^~~
foo.cc:11:20: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     rep(i,1,n)scanf("%s",a[i]+1);
               ~~~~~^~~~~~~~~~~~~

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