i = 10 * 1.5
n = float(input())
if n <= i:
    m = n / 1.5
elif n > i:
    f = n - i
    m = 10 * 1.5 + f / 2.5
print("%.2f" % m)
/**************************************************************
	Problem: 1052
	User: lzk
	Language: Python
	Result: Wrong Answer
****************************************************************/