#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#include <algorithm>
#include <math.h>
#include <map>
#define pb push_back
#define mp make_pair
#define fi first
#define se second
using namespace std;
ifstream in("input.txt");
ofstream out("output.txt");
typedef long long ll;
int n;
int v[100001];
int dp[100001][4][4][4][4];
int funz(int a, int b, int c){
if(b==3&&c==3){
b=a;
c=a;
}
else if(c==3&&b!=3){
c=a;
}
if(a!=b&&b!=c&&a!=c)return 3;
if(a==b&&b!=c)return 2;
if(a==b&&b==c)return 1;
if(a==c&&c!=b)return 2;
return 2;
}
int rec(int pos, int a1, int a2, int b1, int b2){
if(pos==n)return 0;
if(dp[pos][a1][a2][b1][b2]!=-1)return dp[pos][a1][a2][b1][b2];
dp[pos][a1][a2][b1][b2]=max(funz(v[pos],a1,a2)+rec(pos+1,v[pos],a1,b1,b2),funz(v[pos],b1,b2)+rec(pos+1,a1,a2,v[pos],b1));
//cout<<"pos "<<pos<<" a1 "<<a1<<" a2 "<<a2<<" b1 "<<b1<<" b2 "<<b2<<" dp "<<dp[pos][a1][a2][b1][b2]<<endl;
return dp[pos][a1][a2][b1][b2];
}
int main(){
cin>>n;
for(int i=0;i<n;i++){
char g; cin>>g;
if(g=='B')v[i]=0;
else if(g=='M')v[i]=1;
else if(g=='F')v[i]=2;
}
for(int i=0;i<n;i++){
for(int y=0;y<4;y++){
for(int j=0;j<4;j++){
for(int u=0;u<4;u++){
for(int h=0;h<4;h++){
dp[i][y][j][u][h]=-1;
}
}
}
}
}
cout<<rec(0,3,3,3,3);
}
