Summation of a abnormal complex series

In a recent cmpetitive programming contest, I faced a problem. I have found:

1 + (1 + 2) + (1 + 2 + 3) + ........... + (1 + 2 + .... + n) = ?

I tried using 2 nested loops:

for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
        sum += j;
    }
}

But it is showing Time Limit Error.

>Solution :

You can solve the problem using an equation:

Apply this equation to your code. It will work perfectly with time complexity: O(1).