"use strict";

const main = arg => {
    const input = arg.trim().split("\n");
    let strS = input[0].split('');
    let strT = input[1].split('');
    let result = 0;
    
    for(let i = 0; i < strS.length; i++){
        if(strS[i] !== strT[i]) {
            result += 1;
        }
    }
    
    console.log(result)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));  