process.stdin.resume();
process.stdin.setEncoding('utf8');

var input_lines = [];
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});

reader.on('line', (line) => {
  input_lines.push(line);
});

reader.on('close', () => {
  //console.log(input_lines);
  var line_one = input_lines[0];
  var line_two = input_lines[1];

  var one_split = line_one.split('');
  var two_split = line_two.split('');

  var correct_times = 0;

  //console.log(line_one);
  //console.log(line_two);
  //console.log(one_split);
  //console.log(two_split);

  for(var i = 0; i < one_split.length; i ++){
    if(one_split[i] != two_split[i]){
      correct_times ++;
    }
  }

  console.log(correct_times);

});
