#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int long long
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define FORD(i, a, b) for (int i = (b); i >= (a); i --)
#define REP(i, a) for (int i = 0; i < (a); ++i)
#define REPD(i, a) for (int i = (a) - 1; i >= 0; --i)
#define MASK(i) (1LL << (i))
#define BIT(x, i) (((x) >> (i)) & 1)
constexpr ll LINF = (1ll << 60);
constexpr int INF = (1ll << 30);
constexpr int Mod = 1e9 + 7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
/*
Phu Trong from Nguyen Tat Thanh High School for gifted student
*/
template <class X, class Y>
bool minimize(X &x, const Y &y){
X eps = 1e-9;
if (x > y + eps) {x = y; return 1;}
return 0;
}
template <class X, class Y>
bool maximize(X &x, const Y &y){
X eps = 1e-9;
if (x + eps < y) {x = y; return 1;}
return 0;
}
#define MAX 100005
int dp[2][4][4][4][4];
//0: not consider yet
//1: fish
//2: meat
//3: bread
int N;
string s;
int conv(char c){
if(c == 'F') return 1;
if(c == 'M') return 2;
if(c == 'B') return 3;
return 0;
}
int cost(int a, int b, int c){
if(a * b * c == 0){
int cnt = 0;
cnt += (a == 0);
cnt += (b == 0);
cnt += (c == 0);
if(cnt == 1) return (b != c) + 1;
if(cnt == 2) return 1;
}
if(a != b && b != c && c != a) return 3;
if(a != b || b != c || c != a) return 2;
return 1;
}
void process(void){
cin >> N >> s;
memset(dp, -0x3f, sizeof dp);
dp[0][0][0][0][0] = 0;
REP(i, N){
memset(dp[1], -0x3f, sizeof dp[1]);
REP(l_semi, 4) REP(l_last, 4) REP(r_semi, 4) REP(r_last, 4){
int v = conv(s[i]);
int &ret = dp[0][l_semi][l_last][r_semi][r_last];
maximize(dp[1][l_last][v][r_semi][r_last], ret + cost(l_semi, l_last, v));
maximize(dp[1][l_semi][l_last][r_last][v], ret + cost(r_semi, r_last, v));
}
swap(dp[0], dp[1]);
}
int ans = 0;
REP(l_semi, 4) REP(l_last, 4) REP(r_semi, 4) REP(r_last, 4){
maximize(ans, dp[0][l_semi][l_last][r_semi][r_last]);
}
cout << ans;
}
signed main(){
#define name "Whisper"
cin.tie(nullptr) -> sync_with_stdio(false);
// freopen(name".inp", "r", stdin);
// freopen(name".out", "w", stdout);
process();
return (0 ^ 0);
}
