'use strict';

let count = 0;
// let answer;
function main(input) {
    //const num = parseInt(input)
	const str_list = input.replace("\n"," ").split(" ");
  	for (let i = 0; i < str_list[0].length; i++){
       if(str_list[0][i] !== str_list[1][i]){
         count = count + 1
       }
    }
	console.log(count)
}
 
main(require('fs').readFileSync('/dev/stdin', 'utf8'))