In the program below, we've used an `if...else`

statement in combination with a while loop to calculate the sum of natural numbers up to `num`.

## Source Code

```
# Sum of natural numbers up to num
num = 16
if num < 0:
print("Enter a positive number")
else:
sum = 0
# use while loop to iterate until zero
while(num > 0):
sum += num
num -= 1
print("The sum is", sum)
```

**Output**

The sum is 136

**Note:** To test the program for a different number, change the value of `num`.

Initially, the `sum` is initialized to 0. And, the number is stored in variable `num`.

Then, we used the `while`

loop to iterate until `num` becomes zero. In each iteration of the loop, we have added the `num` to `sum` and the value of `num` is decreased by 1.

We could have solved the above problem without using a loop by using the following formula.

n*(n+1)/2

For example, if **n = 16**, the sum would be **(16*17)/2 = 136**.

**Your turn:** Modify the above program to find the sum of natural numbers using the formula below.