JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Алгоритм</title>
</head>
<body>
  <input id="n" type="text" placeholder="Число n">
  <input id="k" type="text" placeholder="Число k">
  <input id="m" type="text" placeholder="Число m">
  <button onclick="" id="search">Search</button>
  <script>
    var n = document.getElementById('n');
    var k = document.getElementById('k');
    var m = document.getElementById('m');
    var search = document.getElementById('search');
    var res = 0;
    search.onclick = function(){
      for (var i = 0; i < n.length; i++) {
          if (n.charAt(i) == m){
          	res = res + 1;
          }
      }
      for (var i = 0; i < k.length; i++) {
          if (k.charAt(i) == m){
          	res = res + 1;
          }
      }
      alert(res);
    }
  </script>
</body>
</html>