"use strict";

const main = arg => {
   const input = arg.trim().split("\n").map(el => Array.from(el));
   let n = 0;
   input[0].forEach((el, i) => {
   	if(el !== input[1][i]) n++;
   });
  console.log(n);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));