"use strict"
function main(arg) {
    let S = arg.trim().split("\n")[0]
    let T = arg.trim().split("\n")[1]
    let ans = 0
    for (let i = 0; i < S.length; i++) {
        if (S[i] != T[i]) {
            ans++
        }
    }
    console.log(ans)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));