Sum Values at Even Positions

Source: Phước Hưng OJ

Version: Phuoc Hung OJ Extended

Problem Statement

Sum values at 1-based positions 2,4,6,...; the positions, not the values, must be even.

Input

n in [0,100000] followed by n integers in [-1000000,1000000].

Output

64-bit sum; print 0 if no even position exists.

Subtasks

• Subtask 1 (20%): $n\le5$, $|a_i|\le10$.
• Subtask 2 (30%): $n\le100$, $|a_i|\le1000$.
• Subtask 3 (50%): $n\le100000$, $|a_i|\le10^6$.

Examples

Example 1

Input

5
8 -3 4 7 2

Output

4

Explanation

Take positions 2 and 4: -3+7=4.

Example 2

Input

1
6

Output

0

Explanation

There is no even position.