109B - Lucky Probability - CodeForces Solution

pl, pr, vl, vr, k = map(int, input().split())
p, x = [0, 1e9], 0


def g(x):
    if x < 1e9:
        p.append(x)
        g(10 * x + 4), g(10 * x + 7)


g(4), g(7), p.sort()


def f(a, b, c, d):
    res = max(0, min(b, d) - max(a, c) + 1)
    return res


for i in range(1, len(p) - k):
    l1 = p[i - 1] + 1
    l2 = p[i]
    l3 = p[i + k - 1]
    l4 = p[i + k] - 1
    x += f(l1, l2, vl, vr) * f(l3, l4, pl, pr) + f(l1, l2, pl, pr) * f(l3, l4, vl, vr)
    if k == 1 and max(vl, pl) <= l2 <= min(vr, pr):
        x -= 1
print(1. * x / ((pr - pl + 1) * (vr - vl + 1)))