text: // serial.c
  
  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  
  int valid_serial( char *psz )
  {
     size_t len = strlen( psz );
     unsigned total = 0;
     size_t i;
  
     if( len < 10 )
        return 0;
  
     for( i = 0; i < len; i++ )
     {
        if(( psz[i] < '0' ) || ( psz[i] > 'z' ))
           return 0;
  
        total += psz[i];
     }
  
     if( total % 853 == 83 )
        return 1;
  
     return 0;
  }
  
  int validate_serial()
  {
     char serial[ 24 ];
  
     fscanf( stdin, "%s", serial );
  
     if( valid_serial( serial ))
        return 1;
     else
        return 0;
  }
  
  int do_valid_stuff()
  {
     printf("The serial number is valid!\n");
     // do serial-restricted, valid stuff here.
     exit( 0 );
  }
  
  int do_invalid_stuff()
  {
     printf("Invalid serial number!\nExiting\n");
     exit( 1 );
  }
  
  int main( int argc, char *argv[] )
  {
     if( validate_serial() )
        do_valid_stuff(); // 0x0804863c
     else
             do_invalid_stuff();
  
     return 0;
  }
  