/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception{
                Scanner in = new Scanner(System.in);
                int n = in.nextInt();
                int[] a = new int[n];
                for (int i=0; i<n; i++){
                     a[i] = in.nextInt();
                }
                double sum=0;
                int f=1;
                for (int i=1; i<=n; i++){
                     f*=i;
                     double sign = (i%2==0) ? 1 : -1;
                     sum+=(sign*a[i-1])/f;
                }
                System.out.println(sum);
	}
}

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Mgo1Cjg=

2
5
8