#include <iostream>
#include <algorithm>
#include <vector>
#include <cstring>
#include <set>
using namespace std;
const int maxn = 55000;
const int INF = 2e9;

string s;
int n;
int dp[maxn][4][4][4][4];

int score(int a, int b, int c) {
    set<int> st;
    if(a != 4) {
        st.insert(a);
    }
    if(b != 4) {
        st.insert(b);
    }
    if(c != 4) {
        st.insert(c);
    }

    return (int) st.size();
}
int color(char c) {
    if(c == 'R') return 0;
    if(c == 'G') return 1;
    return 2;
}
int rec(int at, int L1, int L2, int R1, int R2) {
    if(at == n) {
        return 0;
    }
    if(dp[at][L1][L2][R1][R2] != -1) {
        return dp[at][L1][L2][R1][R2];
    }
    int res = -INF;
    int score_left = score(L1, L2, color(s[at]));
    res = max(res, rec(at + 1, L2, color(s[at]), R1, R2) + score_left);

    int score_right = score(R1, R2, color(s[at]));
    res = max(res, rec(at + 1, L1, L2, R2, color(s[at])) + score_right);

    dp[at][L1][L2][R1][R2] = res;
    return res;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin >> n >> s;
    memset(dp, -1, sizeof dp);

    cout << rec(0, 4, 4, 4, 4) << endl;


    return 0;
}