process.stdin.resume();
process.stdin.setEncoding('utf8');
var lines = [];
var reader = require('readline').createInterface({
    input: process.stdin,
    output: process.stdout,
    terminal: false
});
reader.on('line', (line) => {
    lines.push(line);
});
reader.on('close', () => {
    const s = lines[0];
    const t = lines[1];

    // diff : str, str -> int
    const diff = (s, t) => {
        let cnt = 0;
        for (let i = 0; i < s.length; i++) {
            if (s[i] !== t[i]) {
                cnt++;
            }
        }
        return cnt;
    }
    console.log(diff(s, t));
});
