#include <iostream>
using namespace std;
int n , r , maxim , m;
long long p;
int main()
{
cin>>n;
r=n;
p=1;
maxim=-1;
while(r!=0)
{
m = (n / (10 * p)) * p + n % p;
if(m != 0)
if(n % m > maxim)
maxim = n % m;
r = n / (10 * p);
p = p * 10;
}
cout << maxim;
return 0;
}