'use strict';

function Main(input) {
    const tmp = input.trim().split('\n');
    const S = tmp[0];
    const T = tmp[1];

    let count = 0;
    for (let i = 0; i < S.length; i++) {
        if (S[i] !== T[i]) {
            count += 1;
        }
    }
    console.log(count);

    return;
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'));