CF96B Lucky Numbers (easy)

Petya loves lucky numbers. Everybody knows that positive integers are lucky if their decimal representation doesn't contain digits other than $ 4 $ and $ 7 $ . For example, numbers $ 47 $ , $ 744 $ , $ 4 $ are lucky and $ 5 $ , $ 17 $ , $ 467 $ are not. Lucky number is super lucky if it's decimal representation contains equal amount of digits $ 4 $ and $ 7 $ . For example, numbers $ 47 $ , $ 7744 $ , $ 474477 $ are super lucky and $ 4 $ , $ 744 $ , $ 467 $ are not. One day Petya came across a positive integer $ n $ . Help him to find the least super lucky number which is not less than $ n $ .

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