#include <bits/stdc++.h>
using namespace std;
#define sz(s) (int)s.size()
const int N = 200005;
int n;
string s;
map <char, int> m;
map <int,bool> m1;
int main(){
cin >> n >> s;
vector <int> a;
vector <vector <vector <vector <vector<int>>>>> dp(2, vector <vector <vector<vector <int>>>> (4, vector <vector<vector <int>>> (4, vector<vector <int>> (4, vector <int> (4,-1)))));
int cnt = 0;
for(int i = 0; i < n; i++){
if(m.find(s[i]) == m.end()){
cnt++;
m[s[i]] = cnt;
}
a.push_back(m[s[i]]);
}
dp[0][0][0][0][0] = 0;
for(int i = 0; i < n; i++){
for(int a1 = 0; a1 < 4; a1++){
for(int a2 = 0; a2 < 4; a2++){
for(int b1 = 0; b1 < 4; b1++){
for(int b2 = 0; b2 < 4; b2++){
if(dp[0][a1][a2][b1][b2] == -1) continue;
m1.clear();
int k = 0;
m1[0] = 1;
if(m1.find(a1) == m1.end()) k++, m1[a1] = 1;
if(m1.find(a2) == m1.end()) k++, m1[a2] = 1;
if(m1.find(a[i]) == m1.end()) k++, m1[a[i]] = 1;
if(a2 == 0){
dp[1][a1][a[i]][b1][b2] = dp[0][a1][a2][b1][b2] + k;
}
else {
dp[1][a2][a[i]][b1][b2] = dp[0][a1][a2][b1][b2] + k;
}
k = 0;
m1.clear();
m1[0] = 1;
if(m1.find(b1) == m1.end()) k++, m1[b1] = 1;
if(m1.find(b2) == m1.end()) k++, m1[b2] = 1;
if(m1.find(a[i]) == m1.end()) k++, m1[a[i]] = 1;
if(b2 == 0){
dp[1][a1][a2][b1][a[i]] = dp[0][a1][a2][b1][b2] + k;
}
else {
dp[1][a1][a2][b2][a[i]] = dp[0][a1][a2][b1][b2] + k;
}
}
}
}
}
dp[0] = dp[1];
}
int ans = 0;
for(int a1 = 0; a1 < 4; a1++){
for(int a2 = 0; a2 < 4; a2++){
for(int b1 = 0; b1 < 4; b1++){
for(int b2 = 0; b2 < 4; b2++){
ans = max(ans,dp[0][a1][a2][b1][b2]);
}
}
}
}
cout << ans << '\n';
}
