View Compilation Error

0_0_39464992_32757.cpp: In function 'int main()':
0_0_39464992_32757.cpp:25:24: error: could not convert 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::substr(size_type, size_type) const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long long unsigned int](((std::__cxx11::basic_string<char>::size_type)i), a.std::__cxx11::basic_string<char>::size())' from 'std::__cxx11::basic_string<char>' to 'bool'
   25 |             if(b.substr(i,a.size()))
      |                ~~~~~~~~^~~~~~~~~~~~
      |                        |
      |                        std::__cxx11::basic_string<char>