n = int(input())
b = n // 100 % 10
s = n // 10 % 10
g = n // 1 % 10
x = b + s
y = s + g
print("%.2f" % (x / y))
/**************************************************************
	Problem: 1608
	User: admin
	Language: Python
	Result: Accepted
	Time:100 ms
	Memory:34244 kb
****************************************************************/