function main(input) {
  const s = input[0].split("");
  const t = input[1].split("");

  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').trim().split("\n"));