function Main (input) {
    input = input.split("\n");
    // var z = parseInt(input[0], 10);

    var s = input[0]
    var t = input[1]
    var z = []

    var cnt = 0
    var i = 0
    s.split("").forEach(char_s => {
        var char_t = t.split("")[i]

        if (char_s !== char_t) {
            cnt++
            z.push(char_t)
        } else {
            z.push(char_s)
        }

        var y = z.join("")

        if (t === y) {
            return
        }

        i++
    })

    console.log(cnt)
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));