View Compilation Error

0_0_28175446_11593.cpp: In function 'int main()':
0_0_28175446_11593.cpp:14:16: error: 's' was not declared in this scope
     while(cin>>s)    //用于多次输入字符串
                ^
0_0_28175446_11593.cpp:16:24: error: cannot convert 'std::string {aka std::basic_string<char>}' to 'const char*' for argument '1' to 'size_t strlen(const char*)'
         length=strlen(a);
                        ^