<!DOCTYPE html><meta charset='utf-8'>∑ [
<input type="number" step=1 value=2> ,
<input type="number" step=1 value=3> ] = <data></data>
<script>
setInterval( () => {
var [ n1, n2 ] = document.querySelectorAll( 'input' );
document.querySelector( 'data' ).textContent = ( function add( a, b ) {
return a > b ? NaN : ( a === b ? a : a + add( a + 1, b ) );
} )( ~~n1.value, ~~n2.value );
}, 200 );
</script>