var input = document.querySelector("input");
var output = document.querySelector("p");
var button = document.querySelector("button");

button.addEventListener("click", function (evt) {
    var seconds = input.value;
    output.innerHTML = seconds;
    var counter = window.setInterval(function(){
      seconds = seconds - 1;
      if(seconds <= 0){
        window.clearInterval(counter);
      }
      output.innerHTML = seconds;
    }, seconds*1000);
});

<label for=num>Enter No of Seconds </label><input type=number id=num></input>
<button>start</button>
<p>
</p>