View Compilation Error

0_0_35625824_3828.cpp: In function 'int main()':
0_0_35625824_3828.cpp:26:36: error: invalid types 'int[std::basic_string<char>::size_type {aka long long unsigned int}]' for array subscript
         cout<<dp[a.size()][b.size()]<<endl;
                                    ^