function main(input) {
  let args = input.split(/\s|\n/);
  let change = 0;
  for (i = 0;i < args[0].length;i++) {
    if (args[0][i] !== args[1][i]) {
     change++;
    }
  }
  console.log(change);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));