n = int(input())
q = n // 1000 % 10
b = n // 100 % 10
s = n // 10 % 10
g = n // 1 % 10
mx = max(g, s, b, q)
print(mx)

/**************************************************************
	Problem: 1718
	User: admin
	Language: Python
	Result: Accepted
	Time:325 ms
	Memory:34244 kb
****************************************************************/