View Compilation Error

0_0_21312728_22706.cpp: In function 'int main(int, const char**)':
0_0_21312728_22706.cpp:29:21: error: cannot convert 'std::string {aka std::basic_string<char>}' to 'const char*' for argument '1' to 'size_t strlen(const char*)'
    int n = strlen(st);
                     ^