number = int(input())
y = np.zeros(number)
t = np.zeros(number)

for i in range(number):
    a = input()
    ix = a.find(' ')
    t[i], y[i] = a[:ix], arr[ix+1:]

summ, counter = 0, 0
for i in range(len(y)):
    for j in range(len(y)):
        if t[i] > t[j]:
            counter += 1
            summ += (int(y[i] > y[j]) + 0.5 * int(y[i] == y[j]))

print("%.6f" % (summ / counter))