foo.cc: In function 'int main()':
foo.cc:9:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.length();i++) if(s[i]=='5') v.push_back(i);
              ~^~~~~~~~~~~
foo.cc:12:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=1;i<v.size();i++) ans-=(ll)(v[i]-v[i-1])*(v[i]-v[i-1]-1)/2; // 减去两个'5'之间的子串个数。 
              ~^~~~~~~~~

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