#include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <unordered_map>
#include <queue>
#include <set>
#include <unordered_set>
#include <complex>
#include <list>
#include <cassert>
#include <chrono>
#include <random>
#include <stack>
#include <iomanip>
#include <fstream>
using namespace std;
#define endl "\n"
// #define int long long
const int INF = 1e9+7;
const int MOD = 1e9+7;
int check(int a, int b, int c){
unordered_set<int> st;
if(a) st.insert(a);
if(b) st.insert(b);
if(c) st.insert(c);
return (int)st.size();
}
void solve(){
int n;
string s;
cin >> n >> s;
vector<int> a(n);
for(int i = 0; i < n; i++){
if(s[i] == 'M') a[i] = 1;
if(s[i] == 'F') a[i] = 2;
if(s[i] == 'B') a[i] = 3;
}
vector<vector<vector<vector<int>>>> dp1(4, vector<vector<vector<int>>>(4, vector<vector<int>>(4, vector<int>(4, -1))));
vector<vector<vector<vector<int>>>> dp2(4, vector<vector<vector<int>>>(4, vector<vector<int>>(4, vector<int>(4, -1))));
dp1[0][0][0][0] = 0;
for(int i = 0; i < n; i++){
for(int a1=0; a1<4; a1++) for(int b1=0; b1<4; b1++) for(int a2=0; a2<4; a2++) for(int b2=0; b2<4; b2++){
dp2[a1][b1][a2][b2] = -1;
}
for(int a1 = 0; a1 < 4; a1++){
for(int b1 = 0; b1 < 4; b1++){
for(int a2 = 0; a2 < 4; a2++){
for(int b2 = 0; b2 < 4; b2++){
if(dp1[a1][b1][a2][b2] == -1) continue;
int cnt1 = check(a1, b1, a[i]);
dp2[b1][a[i]][a2][b2] = max(dp2[b1][a[i]][a2][b2], dp1[a1][b1][a2][b2]+cnt1);
int cnt2 = check(a2, b2, a[i]);
dp2[a1][b1][b2][a[i]] = max(dp2[a1][b1][b2][a[i]], dp1[a1][b1][a2][b2]+cnt2);
}
}
}
}
dp1 = dp2;
}
int sol = 0;
for(int a1 = 0; a1 < 4; a1++){
for(int b1 = 0; b1 < 4; b1++){
for(int a2 = 0; a2 < 4; a2++){
for(int b2 = 0; b2 < 4; b2++){
sol = max(sol, dp1[a1][b1][a2][b2]);
}
}
}
}
cout << sol << endl;
}
int32_t main(){
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int t = 1;
// cin >> t;
while(t--) solve();
return 0;
}
