def solve(n):
    cum = 1
    for i in range(1, n):
        s = i * (i + 1) // 2
        t = ((s * s - sum(j**2 for j in range(1, i + 1)))/i + 2)/(i + 1)
        cum += t
    return cum

n = int(input())
print(solve(n))