#include <bits/stdc++.h>
using namespace std;
int main()
{
int n;
string s;
cin>>n>>s;
int x = 0;
while(s[x] != 'x')
x++;
int sum = 0;
for(int i = n - 2; i >= 0; i--)
{
if(s[i] == 'x')
continue;
int a = s[i] - '0';
if((n - 2 - i) % 2 == 0)
{
a *= 2;
if(a > 9)
a = a / 10 + a % 10;
}
sum += a;
//cout<<a<<" ";
}
//cout<<endl<<sum<<" ";
sum *= 9;
sum %= 10;
//cout<<sum<<" "<<endl;
if(x == n - 1)
{
cout<<sum<<endl;
return 0;
}
for(int a = 0; a < 10; a++)
{
int sum2 = sum;
sum2 = (sum2 + ((a * 9) % 10)) % 10;
if(sum2 == s[n - 1] - '0')
{
cout<<a<<endl;
return 0;
}
}
return 0;
}
