Petya loves lucky numbers. Everybody knows that lucky numbers are positive integers whose decimal representation contains only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.Petya loves lucky numbers. Everybody knows that lu
阅读背景：
CodeForces 122G Lucky Array（一脸懵逼的树状数组）
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