int DoubleLetter(string str) {
   char prev ;
   int i, len ;

   prev = IthChar(str,0) ;
   len = StringLength(str) ;

   for (i = 1 ; i < len ; i++){
      if (IthChar(str,i) == prev) {
	 return(i-1) ;
      } else {
	 prev = IthChar(str,i) ;
      }
   }
   return(-1) ;
}