#include <iostream>
#include <string>
#include <algorithm>
#include <cstring>
using namespace std;
int n;
string s;
int ans;

int cal[9999][4][4][4][4];
bool caltrue[9999][4][4][4][4];// initialized as false

bool bb[4];
int score(int a=1, int b=1, int c=2) {
    memset(bb, false, sizeof(bb));
    bb[a] = true;
    bb[b] = true;
    bb[c] = true;
    int res = 0;
    for (int i = 1; i <= 3; ++i) res += bb[i];
    return res;
    // return __builtin_popcount(((1 << a) | (1 << b) | (1 << c)) >> 1);
}

int dp(int f, char a, char b, char c, char d){
    if (f >= n) return 0;

    if(caltrue[f][a][b][c][d]) return cal[f][a][b][c][d];

    // if(score(a, b, s[f])>score(c, d, s[f])

    int x = dp(f + 1, s[f], a, c, d) + score(a, b, s[f]);
    int y = dp(f + 1, a, b, s[f], c) + score(c, d, s[f]);

    caltrue[f][a][b][c][d]=true;
    if (x > y){
        cal[f][a][b][c][d]=x;
        return x;
    }
    else {
        cal[f][a][b][c][d]=y;
        return y;
    }

}


int main() {
    cin >> n >>s;
    for (int i = 0; i < n; ++i) {
        if (s[i] == 'M') s[i] = 1;
        if (s[i] == 'F') s[i] = 2;
        if (s[i] == 'B') s[i] = 3;
    }
    for (int f = n - 1; f >= 0; --f) {
        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) {
                        int x = cal[(f + 1) % 2][s[f]][a][c][d] + score(a, b, s[f]);
                        int y = cal[(f + 1) % 2][a][b][s[f]][c] + score(c, d, s[f]);

                        cal[f % 2][a][b][c][d] = max(x, y);
                        /* int x = dp(f + 1, s[f], a, c, d) + score(a, b, s[f]);
                        int y = dp(f + 1, a, b, s[f], c) + score(c, d, s[f]); */
                    }
                }
            }
        }
    }
    cout << cal[0][0][0][0][0];
}