#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<queue>
using namespace std;
long long n,a[102201],ans;
double num; 	
int main()
{ 
    cin>>n;
    for(int i=1;i<=n;i++)
    {
    	cin>>a[i];

	}
		if((a[n]-a[1])%(n-1)==0)
    	{
    		cout<<(a[n]-a[1])/(n-1)<<endl;
		}
		else
		{
			num= ((double)(a[n])-(double)(a[n]))/(n-1);
			printf("%.3lf\n",num);
		}
	
	return 0;
}