#include<bits/stdc++.h>
using namespace std;
typedef long long int lli;
int solve(string s, int n)
{
map <char, int> m;
int i = 0, j = n - 1;
for (; i <= j; i++, j--)
{
m[s[i]] = i;
m[s[j]] = j;
}
typedef map <char, int> :: iterator mapitr;
int min = INT_MAX;
int max = 0;
for (mapitr it = m.begin(); it != m.end(); it++)
{
int cur = it -> second;
if (cur < min) min = cur;
if (cur > max) max = cur;
}
return max - min + 1;
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n;
cin >> n;
string s;
cin >> s;
cout << solve(s, n) << endl;
return 0;
}
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