#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
#include <cstdio>
#include <sys/resource.h>
using namespace std;
#ifndef ONLINE_JUDGE
#define dbg(x) cerr << #x <<" "; print(x); cerr << endl;
#else
#define dbg(x)
#endif
void print(long long t) {cerr << t;}
void print(int t) {cerr << t;}
void print(string t) {cerr << t;}
void print(char t) {cerr << t;}
void print(double t) {cerr << t;}
void print(long double t) {cerr << t;}
void print(unsigned long long t) {cerr << t;}
template <class T, class V> void print(pair <T, V> p);
template <class T> void print(vector <T> v);
template <class T> void print(set <T> v);
template <class T, class V> void print(map <T, V> v);
template <class T> void print(multiset <T> v);
template <class T> void print(deque <T> v);
template <class T, class V> void print(pair <T, V> p) {cerr << "{"; print(p.first); cerr << ","; print(p.second); cerr << "}";}
template <class T> void print(vector <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T> void print(set <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T> void print(multiset <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T, class V> void print(map <T, V> v) {cerr << "[ "; for (auto i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T> void print(queue <T> q) {cerr << "[ "; while(!q.empty()) {print(q.front()); cerr << " "; q.pop();} cerr << "]";}
template <class T> void print(priority_queue <T> q) {cerr << "[ "; while(!q.empty()) {print(q.top()); cerr << " "; q.pop();} cerr << "]";}
template <class T> void print(deque <T> v) {cerr << "[ "; for (T i : v) {print(i); cerr << " ";} cerr << "]";}
template <class T> void print(int n__, T arr[]) {cerr << "[ "; for(int i = 0; i <= n__; i++) {print(arr[i]); cerr << " ";} cerr << "]\n";}
#define pb push_back
#define YES cout << "YES\n"
#define NO cout << "NO\n"
#define noAnsw cout << -1 << '\n'
#define ll long long
#define ppb pop_back
#define OK cout << "OK" << endl;
#define ld long double
#define all(v) (v).begin(), (v).end()
#define MP make_pair
#define PII pair <int, int>
#define endl '\n'
#define ull unsigned long long
#define PLL pair <ll, ll>
#define PIL pair <int, ll>
#define PLI pair <ll, int>
#define rall(v) (v).rbegin(), (v).rend()
#define priora priority_queue
#define urishOK cout << "urishOK\n";
void setIO(string name = "") {
if (!name.empty()) {
freopen((name + ".in").c_str(), "r", stdin);
freopen((name + ".out").c_str(), "w", stdout);
}
}
void fastIO(){
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
}
void setPrecision(int x){
if(x == 0){
return;
}
cout.setf(ios::fixed);
cout.precision(x);
return;
}
void increaseStack(rlim_t stackSize) {
struct rlimit rl;
int result;
result = getrlimit(RLIMIT_STACK, &rl);
if (result == 0) {
if (rl.rlim_cur < stackSize) {
rl.rlim_cur = stackSize;
result = setrlimit(RLIMIT_STACK, &rl);
if (result != 0) {
cerr << "Error setting stack size." << endl;
}
}
}
}
const int inf = 1e9 + 5, LOG = 20;
const ll inf64 = 1e18 + 5, mod = 1e9 + 7, wmod = 998244353;
const double epsilon = 0.000001, pi = 3.14159265359;
const int N = 1e5 + 5;
int n, dp[N][4][4][4][4];
string s;
int calc(int x, int y, int z){
set <int> st;
if(x) st.insert(x);
if(y) st.insert(y);
if(z) st.insert(z);
return st.size();
}
void solve(int TST_NUM){
cin >> n >> s;
for(int i = 0; i <= n; i++){
for(int l2 = 0; l2 < 4; l2++){
for(int l1 = 0; l1 < 4; l1++){
for(int r2 = 0; r2 < 4; r2++){
for(int r1 = 0; r1 < 4; r1++){
dp[i][l2][l1][r2][r1] = -inf;
}
}
}
}
}
dp[0][0][0][0][0] = 0;
for(int i = 1; i <= n; i++){
int k;
if(s[i - 1] == 'M') k = 1;
if(s[i - 1] == 'F') k = 2;
if(s[i - 1] == 'B') k = 3;
for(int l2 = 0; l2 < 4; l2++){
for(int l1 = 0; l1 < 4; l1++){
if(l2 && !l1) continue;
for(int r2 = 0; r2 < 4; r2++){
for(int r1 = 0; r1 < 4; r1++){
if(r2 && !r1) continue;
//kpcnenq dzaxin
dp[i][l1][k][r2][r1] = max(dp[i][l1][k][r2][r1], dp[i - 1][l2][l1][r2][r1] + calc(l2, l1, k));
//kpcnenq ajin
dp[i][l2][l1][r1][k] = max(dp[i][l2][l1][r1][k], dp[i - 1][l2][l1][r2][r1] + calc(r2, r1, k));
}
}
}
}
}
int answ = 0;
for(int l2 = 0; l2 < 4; l2++){
for(int l1 = 0; l1 < 4; l1++){
for(int r2 = 0; r2 < 4; r2++){
for(int r1 = 0; r1 < 4; r1++){
answ = max(answ, dp[n][l2][l1][r2][r1]);
}
}
}
}
cout << answ << endl;
}
int main(){
setPrecision(1);
fastIO();
//increaseStack(64 * 1024 * 1024);
//setIO("file");
int Tests = 1;
//cin >> Tests;
int TST_NUM = 1;
while(Tests--){
solve(TST_NUM++);
}
return 0;
}
//mcmcmcmcmcmcmcmcmcmcmc
//mcmcmcmcmcmcmcmcmcmcmc
//mcmcmcmcmcmcmcmcmcmcmc
//mcmcmcmcmcmcmcmcmcmcmc
//matsak
