n = int(input()) ans = 0 if n <= 2: ans = 6 elif n <= 10: ans = 6 + (n - 2) * 1.8 else: ans = 6 + 8 * 1.8 + (n - 10) * 2.7 if int(ans) == ans: print(int(ans)) else: print(round(ans, 1))