Sum of First N Numbers
Add up 1 through N using the closed-form formula.
Input format
A single line with a non-negative integer N.
Output format
Print the sum 1 + 2 + ... + N. Use the formula N * (N + 1) / 2.
Constraints
- Values fit in a 64-bit signed integer
- Trailing whitespace and a trailing newline are ignored by the judge
Read from stdin, write to stdout. Sample cases below show the exact format.
Sample cases
Example 1
Input
5Expected output
15Example 2
Input
10Expected output
55