$("#calculate").click(function(){ var n = $('#n').val(); var r = $('#r').val(); var computation = 0; if(n && n.length > 0 && r && r.length > 0){ computation = combinations(n,r); } $('#result').html(computation); }); $("input").change(function(){ var isInvalid = $('#nDiv').hasClass('is-invalid') || $('#rDiv').hasClass('is-invalid'); if(isInvalid){ $("#calculate").prop("disabled", true); }else{ $("#calculate").removeAttr('disabled'); } });

number of objects to choose from, ${n}$ Number required!

number of objects selected, ${r}$ Number required!