function Main(input){
    input=input.split("\n");  //ここまで必須
    var result=0;
    for (i=0;i<input[0].length;i++){
        if (input[0].slice(i,i+1)!=input[1].slice(i,i+1)){result=result+1}
    }
    console.log(result)
}
Main(require("fs").readFileSync("/dev/stdin","utf8"));