#include <cstdio>
#include <stdio.h>
#include <stdbool.h>
#include <iostream>
#include <map>
#include <vector>
#include <climits>
#include <stack>
#include <string>
#include <queue>
#include <algorithm>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <cmath>
#include <cctype>
#include <bitset>
#include <iomanip>
#include <cstring>
#include <numeric>
#include <cassert>
using namespace std;
#define int long long
#define pii pair<int, int>
#define mp make_pair
#define pb push_back
#define fi first
#define se second
int n, dp[100005][4][4][4][4];
map<char, int> m;
string s;
int calc(int a, int b, int c){
set<int> ss;
if (a)ss.insert(a);
if (b)ss.insert(b);
if (c)ss.insert(c);
return ss.size();
}
int miner(int i, int a1, int a2, int b1, int b2){
if (i==n)return 0;
if (dp[i][a1][a2][b1][b2]!=-1)return dp[i][a1][a2][b1][b2];
return dp[i][a1][a2][b1][b2]=max(calc(a1, a2, m[s[i]])+miner(i+1, a2, m[s[i]], b1, b2), calc(b1, b2, m[s[i]])+miner(i+1, a1, a2, b2, m[s[i]]));
}
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin>>n>>s;
memset(dp, -1, sizeof(dp));
m['M']=1;
m['F']=2;
m['B']=3;
cout<<miner(0, 0, 0, 0, 0);
}
