#include <bits/stdc++.h> using namespace std; int main() { int n; cin>>n; int a[4]; a[0] = n % 10; a[1] = n / 10 % 10; a[2] = n / 100 % 10; a[3] = n / 1000; sort(a,a + 4); cout<<a[3]; return 0; }