let main = (input) => {
    let usrInput = input.split(/\n/)
    let time = 0
    for (let index = 0; index < usrInput[0].length; index++) {
        if (usrInput[0][index]!=usrInput[1][index]) {
            time++
        }
    }
    console.log(time)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'))