#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define du double
#define endl '\n'
#define IOS ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
int main() {
IOS;
int n;
cin>>n;
string s;
cin>>s;
map<char,int > mp;
for(int i=0;i<n;i++) {
mp[s[i]]++;
}
int l =0;
for(int i=0;i<n;i++) {
mp[s[i]]--;
if(mp[s[i]]==0) {
l=i;
break;
}
}
int r=n-1;
for (int i=n-1;i>=0;i--) {
mp[s[i]]--;
if(mp[s[i]]==0) {
r=i;
break;
}
}
// cout <<l<< " "<<r<<endl;
cout <<r-l+1;;
return 0;
}
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