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<input type="text" onkeypress="return onlyNumberKey(event)" required />
<script>
function onlyNumberKey(evt) {
// Only ASCII character in that range allowed
var ASCIICode = (evt.which) ? evt.which : evt.keyCode
if (ASCIICode > 31 && (ASCIICode < 48 || ASCIICode > 57))
return false;
return true;
}
</script>