#include <bits/stdc++.h>
using namespace std;
#define cin(vec) for(auto& i : vec) cin >> i
#define cout(vec) for(auto& i : vec) cout << i << " "; cout << "\n";
#define fast ios::sync_with_stdio(0);cin.tie(0);
#define loop(i,a,b) for (int i = a; i < b; i++)
#define F first
#define S second
#define pb(n) push_back(n)
#define pf(n) push_front(n)
#define dci(d) fixed<<setprecision(d)
#define sp ' '
#define el '\n'
#define all(v) v.begin(),v.end()
#define int long long
int dx[8]= {0,0,1,-1,-1,1,1,-1};
int dy[8]= {-1,1,0,0,-1,1,-1,1};
int const N=1e5+1,M=1e3+1,Mod=1e9+7;
string s;
int n;
int dp[N][5][5][5][5];
int value(int a,int b,int c){
set<int>s;
s.insert(a);
s.insert(b);
s.insert(c);
s.erase(0ll);
return s.size();
}
int trans(char x){
if(x=='M')return 1;
if(x=='B')return 2;
return 3;
}
int rec(int idx, int left_a, int left_b, int right_a, int right_b){
if(idx==n)return 0;
int _idx=idx,lefta=left_a,leftb=left_b,righta=right_a,rightb=right_b;
int &ret=dp[idx][left_a][left_b][right_a][right_b];
if(~ret)return ret;
int c=trans(s[idx]);
int c1=rec(idx+1,left_b,c,right_a,right_b)+value(left_a,left_b,c);
int c2=rec(idx+1,left_a,left_b,right_b,c)+value(right_a,right_b,c);
return dp[idx][lefta][leftb][righta][rightb]=max(c1,c2);
}
void testcase(int h){
cin>>n>>s;
memset(dp,-1,sizeof dp);
cout<<max(rec(1,trans(s[0]),0,0,0)+1,rec(1,0,0,trans(s[0]),0)+1);
}
int32_t main()
{
fast
testcase(1);
// int tc;cin>>tc;for(int i=1;i<=tc;i++)testcase(i);
return 0;
}
