function main(arg) {
    let [S,T] = arg.split("\n")
    
    let count = 0
    for(let i=0; i<S.length; i++) {
        if(S[i]!==T[i]) {
            count++
        }
    }
    console.log(count)

}

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