process.stdin.setEncoding('utf-8')
process.stdin.on('data', (data) => {
    const array = data.trim().split(/\n/)
    let res = 0
    for (let i = 0; i < array[0].trim().length; i++) {
        if (array[0][i] !== array[1][i]){
            res ++
        }
    }
    console.log(res)
})
