i = 10 * 1.5
n = float(input())
if n <= i:
    m = n / 1.5
else:
    f = n - i
    m = 10 + (n - 1.5 * 10) / 2.5
print("%.2f" % m)
/**************************************************************
	Problem: 1052
	User: lzk
	Language: Python
	Result: Accepted
	Time:84 ms
	Memory:34244 kb
****************************************************************/