# import sys; input = sys.stdin.buffer.readline # sys.setrecursionlimit(10**7) from collections import defaultdict con = 10 ** 9 + 7; INF = float("inf") import copy def getlist(): return list(map(int, input().split())) #処理内容 def main(): N = int(input()) ans = N * 7 / 2 print(ans) if __name__ == '__main__': main()