Triangular Number

The n-th triangular number is 1 + 2 + … + n. A formula avoids looping over huge inputs. The 0th triangular number is 0.

Implement triangular_number(n).

Input

n: A non-negative integer.

Output

Return 1 + 2 + … + n.

Constraints

0 ≤ n ≤ 1,000,000,000. The result fits a signed 64-bit integer.

Examples

Input:
n = 10

Expected output:
expected = 55

Input:
long long n = 10;

Expected output:
long long expected = 55;

Input:
long n = 10;

Expected output:
long expected = 55;

Input:
n = 0

Expected output:
expected = 0

Input:
long long n = 0;

Expected output:
long long expected = 0;

Input:
long n = 0;

Expected output:
long expected = 0;