function main(input){
  let [s, t] = input.split('\n').map(v=>v.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'));