import java.util.*;
import java.lang.*;
import java.io.*;
 
class SumNum {
	public static void main (String[] args) throws java.lang.Exception {
		Scanner read = new Scanner(System.in);
		int n = read.nextInt();
		double sum = 0;
		double x1; 
		double x2 = read.nextDouble();
		double x3 = read.nextDouble();
 
		for (int i = 0; i < n-2; i++ ){
			x1 = x2;
			x2 = x3;
		 	x3 = read.nextDouble();
			sum += (x1 + x2 + x3) * x2;
		}
		System.out.print("Сумма = "+sum);
	}
}

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NQowIDAgMiAtNCA3

5
0 0 2 -4 7