var num1, num2; // global variables

function promptIt() {// prompt requires a message string and optional default value
     num1 = parseInt(prompt("Enter the first number:", 1));
     num2 = parseInt(prompt("Enter second number:", 10));
}

function calcIt() {
     var i, sum = 0;

     for (i = num1; i <= num2; i++) sum += i;   // num1 + (num1 + 1) +  ... + num2

     // Display result
     alert("The sum of digits from: "+ num1 + " to " + num2 + " is:\n\n\t " + sum)
}

</SCRIPT>

<FORM>
     Click the button to calculate the sum of the digits:
     <INPUT TYPE="button" VALUE="Calculate" onClick="promptIt(); calcIt();">
</FORM>