#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define int ll
#define fore(a, b, c) for(int a=b; a<c; ++a)
#define vi vector<int>
#define pb push_back
#define all(x) x.begin(), x.end()
#define sz(x) (int)x.size()
#define F first
#define S second
#define ii pair<int,int>
#define dbg(x) cerr << #x << ": " << x << endl
#define raya cerr << "=============" << endl
const int N = 1e5+5;
int n;
string s;
int a[N];
int dp[N][4][4][4][4];
map<char,int> mp = {{'M', 1}, {'F', 2}, {'B', 3}};
int calc(vi x){
set<int> st;
for(int val : x){
st.insert(val);
}
return sz(st);
}
int f(int pos, int p1, int p2, int q1, int q2){
if(pos == n){
return 0;
}
int &res = dp[pos][p1][p2][q1][q2];
if(res > -1){
return res;
}
res = 0;
// mina1
if(p2 == 0){
// no hay nada
int profit = 1;
res = max(res, profit + f(pos+1, p1, a[pos], q1, q2));
}
else if(p1 == 0){
// hay solo uno
int profit = calc({p2, a[pos]});
res = max(res, profit + f(pos+1, p2, a[pos], q1, q2));
}
else {
// ya hay al menos dos
int profit = calc({p1, p2, a[pos]});
res = max(res, profit + f(pos+1, p2, a[pos], q1, q2));
}
// mina2
if(q2 == 0){
// no hay nada
int profit = 1;
res = max(res, profit + f(pos+1, p1, p2, q1, a[pos]));
}
else if(q1 == 0){
// hay solo uno
int profit = calc({q2, a[pos]});
res = max(res, profit + f(pos+1, p1, p2, q2, a[pos]));
}
else {
// ya hay al menos dos
int profit = calc({q1, q2, a[pos]});
res = max(res, profit + f(pos+1, p1, p2, q2, a[pos]));
}
return res;
}
signed main(){
memset(dp, -1, sizeof dp);
cin >> n;
cin >> s;
fore(i, 0, n){
a[i] = mp[s[i]];
}
cout << f(0, 0, 0, 0, 0);
}
