#include <bits/stdc++.h>
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
typedef long long ll;
typedef long double ld;
#define INF 2001001001
#define MOD 1000000007
int N;
string S;
int dp[100001][13][13];
string str[30];
int t[30][3];
void fill(int ind, int a, int b, int c){
t[ind][0]=a;
t[ind][1]=b;
t[ind][2]=c;
}
void precompute(){
str[0]="!!"; fill(0,1,2,3);//t[0]={1,2,3};
str[1]="!M"; fill(1,4,5,6);//t[1]={4,5,6};
str[2]="!F"; fill(2,10,11,12);//t[2]={10,11,12};
str[3]="!B"; fill(3,7,8,9);//t[3]={7,8,9};
str[4]="MM"; fill(4,4,5,6);//t[4]={4,5,6};
str[5]="MF"; fill(5,10,11,12);//t[5]={10,11,12};
str[6]="MB"; fill(6,7,8,9);//t[6]={7,8,9};
str[7]="BM"; fill(7,4,5,6);//t[7]={4,5,6};
str[8]="BF"; fill(8,10,11,12);//t[8]={10,11,12};
str[9]="BB"; fill(9,7,8,9);//t[9]={7,8,9};
str[10]="FM"; fill(10,4,5,6);//t[10]={4,5,6};
str[11]="FF"; fill(11,10,11,12);//t[11]={10,11,12};
str[12]="FB"; fill(12,7,8,9);//t[12]={7,8,9};
}
bool used[4];
int calc(string s){
used[0]=used[1]=used[2]=false;
for (char c:s){
if (c=='M') used[0]=true;
else if (c=='F') used[1]=true;
else if (c=='B') used[2]=true;
}
return used[0]+used[1]+used[2];
}
int main()
{
//ios_base::sync_with_stdio(0);cin.tie(0);
//freopen (".in","r",stdin);
//freopen (".out","w",stdout);
cin>>N>>S;
S='?'+S;
precompute();
for (int i=0;i<=N+1;i++)
for (int j=0;j<=12;j++)
for (int k=0;k<=12;k++)
dp[i][j][k]=-INF;
dp[0][0][0]=0;
for (int i=1;i<=N;i++){
for (int j=0;j<=12;j++)
for (int k=0;k<=12;k++){
int cur;
if (S[i]=='M') cur=0;
else if (S[i]=='F') cur=1;
else cur=2;
//give mine 1
dp[i][t[j][cur]][k]=max(dp[i][t[j][cur]][k],dp[i-1][j][k]+calc(str[j]+S[i]));
dp[i][j][t[k][cur]]=max(dp[i][j][t[k][cur]],dp[i-1][j][k]+calc(str[k]+S[i]));
}
}
int ans=0;
for (int j=0;j<=12;j++)
for (int k=0;k<=12;k++)
ans=max(ans,dp[N][j][k]);
cout<<ans<<endl;
return 0;
}
