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.

Contraintes

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

Exemples

Entrée :
n = 10

Sortie attendue :
expected = 55

Entrée :
long long n = 10;

Sortie attendue :
long long expected = 55;

Entrée :
long n = 10;

Sortie attendue :
long expected = 55;

Entrée :
n = 0

Sortie attendue :
expected = 0

Entrée :
long long n = 0;

Sortie attendue :
long long expected = 0;

Entrée :
long n = 0;

Sortie attendue :
long expected = 0;