#include <bits/stdc++.h>
using namespace std;
#define sp ' '
#define endl '\n'
#define st first
#define nd second
#define pb push_back
#define mp make_pair
#define forn(YY, yy) for(int yy = 0; yy < YY; ++yy)
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef pair<int, int> ii;
const int MOD = 1e9 + 7;
const int INF = 2e9 + 13;
int N;
string S;
map<string, int> conv;
vector<vector<vector<int>>> DN;
int M(int n, string state1, string state2)
{
if(n == N) return 0;
if(DN[n][conv[state1]][conv[state2]] != -1) return DN[n][conv[state1]][conv[state2]];
string newstate1 = state1, newstate2 = state2;
if(newstate1.length() == 2){ newstate1[0] = newstate1[1]; newstate1[1] = S[n]; }
if(newstate1.empty() or newstate1.length() == 1) newstate1 += S[n];
if(newstate2.length() == 2){ newstate2[0] = newstate2[1]; newstate2[1] = S[n]; }
if(newstate2.empty() or newstate2.length() == 1) newstate2 += S[n];
int value1, value2;
if(state1.size() == 2)
{
if(S[n] != state1[0] and S[n] != state1[1] and state1[0] != state1[1]) value1 = 3;
if(S[n] != state1[0] and S[n] != state1[1] and state1[0] == state1[1]) value1 = 2;
if(S[n] == state1[0] and S[n] != state1[1]) value1 = 2;
if(S[n] == state1[1] and S[n] != state1[0]) value1 = 2;
if(S[n] == state1[1] and S[n] == state1[0]) value1 = 1;
}
if(state1.size() == 1)
{
if(S[n] == state1[0]) value1 = 1;
else value1 = 2;
}
if(state1.empty())
{
value1 = 1;
}
if(state2.size() == 2)
{
if(S[n] != state2[0] and S[n] != state2[1] and state2[0] != state2[1]) value2 = 3;
if(S[n] != state2[0] and S[n] != state2[1] and state2[0] == state2[1]) value2 = 2;
if(S[n] == state2[0] and S[n] != state2[1]) value2 = 2;
if(S[n] == state2[1] and S[n] != state2[0]) value2 = 2;
if(S[n] == state2[1] and S[n] == state2[0]) value2 = 1;
}
if(state2.size() == 1)
{
if(S[n] == state2[0]) value2 = 1;
else value2 = 2;
}
if(state2.empty())
{
value2 = 1;
}
//cout << n << sp << state1 << sp << state2 << endl;
//cout << value1 << sp << value2 << endl << endl;
return DN[n][conv[state1]][conv[state2]] = max( M(n+1, newstate1, state2) + value1, M(n+1, state1, newstate2) + value2 );
}
int main(int argc, char **argv)
{
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
cin >> N >> S;
conv["MM"] = 1;
conv["MB"] = 2;
conv["MF"] = 3;
conv["BM"] = 4;
conv["BB"] = 5;
conv["BF"] = 6;
conv["FM"] = 7;
conv["FB"] = 8;
conv["FF"] = 9;
conv["M"] = 11;
conv["F"] = 12;
conv["B"] = 13;
conv[""] = 14;
DN.assign(N+1, vvi(15, vi(15, -1)));
cout << M(0, "", "") << endl;
return 0;
}
//cikisir
