# -*- coding: utf-8 -*-
"""
No.57 ミリオンダイス
https://yukicoder.me/problems/no/57

"""
import sys
from sys import stdin
input = stdin.readline


def main(args):
    N = int(input())
    print(3.5 * N)


if __name__ == '__main__':
    main(sys.argv[1:])