Exciting Energy

Villagers exchange high-fives, and each high-five generates excitement energy. You are given an integer \(n\) (with \(n \geq 2\)) representing the number of villagers, and a sequence of \(n\) positive integers \(s_1, s_2, \dots, s_n\) representing the high-five strength of each villager. The total excitement energy is defined as:

\[ E = \sum_{i=1}^{n-1} s_i \times s_{i+1} \]

Your task is to compute \(E\) given the sequence of strengths.

inputFormat

The input is given from standard input (stdin) and consists of two lines:

• The first line contains a single integer \(n\) (\(n \geq 2\)), the number of villagers.
• The second line contains \(n\) space-separated integers \(s_1, s_2, \dots, s_n\), representing the high-five strengths of the villagers.

outputFormat

Output to standard output (stdout) a single integer, which is the total excitement energy calculated as \(E = \sum_{i=1}^{n-1} s_i \times s_{i+1}\).

## sample

4
1 2 3 4

20

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