n = int(input())
lr = []
for _ in range(n):
    l,r = map(int,input().split())
    lr.append((l,r))

(l0, r0) = lr[0]
ans = 0.0
for i in range(1, n):
    (li, ri) = lr[i]
    sep = ri - li + 1
    l = max(l0, li)
    r = min(r0, ri)
    add = (l - l0) * sep
    if l <= r:
        add += (ri - l + ri - r) * (ri - l - ri + r + 1) / 2
    ans += add / (r0 - l0 + 1) / sep
print(ans + 1.0)