E. Lucky Interval
brute force; math
Petya loves lucky numbers. We all know that lucky numbers are the positive integers whose decimal representations contain only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.One day Petya came across an interval of numbers [a, a + l - 1]. Let F(x) be the number of lucky digits of number x. Find the minimum b (a < b) such, that F(a) = F(b), F(a + 1) = F(b + 1), ..., F(a + l - 1) = F(b + l - 1).
The single line contains two integers a and l (1 ≤ a, l ≤ 109) — the interval's first number and the interval's length correspondingly.
On the single line print number b — the answer to the problem.
Consider that [a, b] denotes an interval of integers; this interval includes the boundaries. That is,