poh.js

(function(stdin) {

  var inputs = stdin.toString().trim().split('\n');
  var nm     = inputs[0].split(' ').map(Number);
  var s      = inputs[1].slice(0, nm[0] + 1).split('');
  var t      = inputs[2].slice(0, nm[1] + 1).split('');
  var hashMap= function(a) {
    return a.reduce(function(map, key) {
      map[key] = map[key] + 1 || 1;
      return map;
    }, {});
  };
  var result = (function(os, ot) {
    return Object.keys(ot)
      .map(function(key) {
        return ot[key] - (os[key] || 0);
      })
      .filter(function(v) {
        return v >= 0;
      })
      .reduce(function(a, b) {
        return a + b;
      });

  })(hashMap(s), hashMap(t));

  console.log(result);

}(require('fs').readFileSync('/dev/stdin', 'utf8')));