View Compilation Error

0_0_27778078_17012.cpp: In function 'int main()':
0_0_27778078_17012.cpp:22:19: error: 'strlen' was not declared in this scope
   int na=strlen(sa),nb=strlen(sb);
                   ^
0_0_27778078_17012.cpp:31:13: error: 'nb' was not declared in this scope
   for(i=0;i<nb;i++)
             ^