#!/usr/bin/env perl
from decimal import *

# yi = 2xi - 1
def merit(y):
    n = len(y)
    
    
    e = Decimal(0)
    for k in range(1,n):
        t = 0
        for i in range(0,n-k):
            t += y[i] * y[i+k]
        e += t ** 2
    f = (n ** 2) / ( 2 * e)
    return(f)

y = [1,-1,1,-1]
print merit(y)