//----------vahagng----------//
#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
using namespace std;
// using namespace __gnu_pbds;
// template <class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
#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, class V> void print(T v[], V n) { cerr << "["; for (int i = 0; i < n; i++) { cerr << v[i] << " "; } cerr << "]"; }
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(unordered_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 << "]"; }
#define ll long long
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define ld long double
#define sz(v) v.size()
#define endl '\n'
const ll inf = 2e18, mod = 1e9 + 7, mod2 = 998244353;
void SetIO(string str = "") {
if (str != "") {
freopen((str + ".in").c_str(), "r", stdin);
freopen((str + ".out").c_str(), "w", stdout);
}
else {
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
}
}
void FastIO() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
}
const int N = 3e5 + 10, M = 2e7 + 10;
mt19937 rnd(chrono::system_clock::now().time_since_epoch().count());
ll binpow(ll a, ll b, ll m) {
ll res = 1;
while (b) {
if (b & 1) {
res = (res * a) % m;
}
b >>= 1;
a = (a * a) % m;
}
return res;
}
ll n, dp[N][4][4][4][4];
string s;
// M - 0
// B - 1
// F - 2
int calc(int a, int b, int c) {
if (!a && !b) return 1;
else if (!a) return (b == c ? 1 : 2);
else if (!b) return (a == c ? 1 : 2);
else {
if(a == b){
return (a == c ? 1 : 2);
}
else {
return (a == c || b == c ? 2 : 3);
}
}
return 0;
}
void solve(int tc) {
cin >> n >> s;
s = "$" + s;
for (int i = 0; i <= n; i++) {
for(int a = 0; a <= 3; a++){
for (int b = 0; b <= 3; b++) {
for (int c = 0; c < 4; c++) {
for (int d = 0; d < 4; d++) {
dp[i][a][b][c][d] = -inf;
}
}
}
}
}
dp[0][0][0][0][0] = 0;
map<char, int> mp;
mp['M'] = 1;
mp['B'] = 2;
mp['F'] = 3;
for (int i = 1; i <= n; i++) {
for (int a = 0; a <= 3; a++) {
for(int b = 0; b <= 3; b++){
for (int c = 0; c <= 3; c++) {
for (int d = 0; d <= 3; d++) {
if (s[i] == 'M') {
dp[i][1][a][c][d] = max(dp[i][1][a][c][d], dp[i - 1][a][b][c][d] + calc(a, b, 1));
dp[i][a][b][1][c] = max(dp[i][a][b][1][c], dp[i - 1][a][b][c][d] + calc(c, d, 1));
}
else if (s[i] == 'B') {
dp[i][2][a][c][d] = max(dp[i][2][a][c][d], dp[i - 1][a][b][c][d] + calc(a, b, 2));
dp[i][a][b][2][c] = max(dp[i][a][b][2][c], dp[i - 1][a][b][c][d] + calc(c, d, 2));
}
else {
dp[i][3][a][c][d] = max(dp[i][3][a][c][d], dp[i - 1][a][b][c][d] + calc(a, b, 3));
dp[i][a][b][3][c] = max(dp[i][a][b][3][c], dp[i - 1][a][b][c][d] + calc(c, d, 3));
}
}
}
}
}
}
ll ans = 0;
for(int a = 0; a <= 3; a++){
for(int b = 0; b <= 3; b++){
for(int c = 0; c <= 3; c++){
for(int d = 0; d <= 3; d++){
ans = max(ans, dp[n][a][b][c][d]);
}
}
}
}
// for (int i = 1; i <= n; i++) {
// for (int a = 0; a <= 3; a++) {
// for (int b = 0; b <= 3; b++) {
// for (int c = 0; c <= 3; c++) {
// for (int d = 0; d <= 3; d++) {
// cout << "dp[" << i << "][" << a << "][" << b << "][" << c << "][" << d << "] = " << dp[n][a][b][c][d] << endl;
// }
// }
// }
// }
// }
cout << ans << endl;
}
void precalc() {
}
int main() {
// SetIO("teamwork");
FastIO();
int test_case = 1;
// cin >> test_case;
precalc();
int cnt = 1;
while (test_case--) {
solve(cnt++);
}
return 0;
}
