View Compilation Error

0_0_21054660_5587.cpp: In function 'int main()':
0_0_21054660_5587.cpp:18:28: error: 'strlen' was not declared in this scope
         int len = strlen(ch);
                            ^
0_0_21054660_5587.cpp:31:10: error: expected primary-expression before 'long'
     n += long long((ch[j] - '0') * pow(10.0, double(k)));
          ^