import java.util.*;
import java.lang.*;
import java.io.*;
class SumNum {
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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