'use strict'

function main(input) {
    let s = String(input.split('\n')[0].split(' ')[0]);
    let t = String(input.split('\n')[1].split(' ')[0]);
    let count = 0;
    for (let i = 0; i < s.length; i++) {
        if (s[i] != t[i]) {
            count++;
        }
    }
    console.log(count);
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));