n = int(input()) li = [False] * 2 + [True] * (n - 1) left, right = 2, 2 acc = left ans = 0 for i in range(2, n + 1): for j in range(2 * i, n + 1, i): if li[j]: li[j] = False def find_next(num): while num < n: num += 1 if li[num]: return num return False while left