/in/foo.cc: In function 'void read()':
/in/foo.cc:15:29: warning: unused variable 'len' [-Wunused-variable]
     for(int i=strlen(cha)-1,len = strlen(cha)-1;i>=0;i--)
                             ^~~
/in/foo.cc:18:29: warning: unused variable 'len' [-Wunused-variable]
     for(int i=strlen(chb)-1,len = strlen(chb)-1;i>=0;i--)
                             ^~~