const Main = input => {
    input = input.split("\n");
    const s = input[0];
    const t = input[1];

    let count = 0;
    for(let i = 0; i < s.length; i++) {
        if(!(s.charAt(i) === t.charAt(i))) {
            count++;
        }
    }
    
    console.log(count);
}
 
Main(require("fs").readFileSync("/dev/stdin", "utf8"));