#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ll long long
#define P pair<int,int>
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<n;i++)
#define all(v) v.begin(),v.end()
#define pb push_back
template<class T>void chmax(T &a,T b){if(a<b)a=b;}
template<class T>void chmin(T &a,T b){if(a>b)a=b;}
constexpr int INF=1000000000000000000;
constexpr int mod=1000000007;
int dx[]={0,1,0,-1},dy[]={1,0,-1,0};
int kaijo[2000010];
int gcd(int a,int b){
if(b==0)return a;
return gcd(b,a%b);
}
int lcm(int a,int b){
return a/gcd(a,b)*b;
}
bool prime(int a){
if(a==1)return false;
for(int i=2;i*i<=a;i++){
if(a%i==0)return false;
}
return true;
}
void init_fact(){
kaijo[0]=1;
for(int i=1;i<=2000000;i++){
kaijo[i]=kaijo[i-1]*i;
kaijo[i]%=mod;
}
}
int modpow(int a,int b){
if(b==0)return 1;
if(b%2)return modpow(a,b-1)*a%mod;
int memo=modpow(a,b/2);
return memo*memo%mod;
}
int comb(int a,int b){
if(!kaijo[0])init_fact();
return kaijo[a]*modpow(kaijo[a-b],mod-2)%mod*modpow(kaijo[b],mod-2)%mod;
}
int inv(int x){
x=modpow(x,mod-2);
return x;
}
bool kosa(double ax,double ay,double bx,double by,double cx,double cy,double dx,double dy){
double ta=(cx-dx)*(ay-cy)+(cy-dy)*(cx-ax);
double tb=(cx-dx)*(by-cy)+(cy-dy)*(cx-bx);
double tc=(ax-bx)*(cy-ay)+(ay-by)*(ax-cx);
double td=(ax-bx)*(dy-ay)+(ay-by)*(ax-dx);
return tc*td<0&&ta*tb<0;
}
int n,a[100010];
string s;
int dp[100010][4][4][4][4];
signed main(){
cin>>n>>s;
rep(i,n+1){
rep(c,4){
rep(d,4){
rep(e,4){
rep(f,4){
dp[i][c][d][e][f]=-INF;
}
}
}
}
}
dp[0][3][3][3][3]=0;
rep(i,n){
if(s[i]=='M')a[i]=0;
if(s[i]=='F')a[i]=1;
if(s[i]=='B')a[i]=2;
}
rep(i,n){
rep(c,4){
rep(d,4){
rep(e,4){
rep(f,4){
set<int>x,y;
x.insert(c);
x.insert(d);
x.insert(a[i]);
x.erase(3);
y.insert(e);
y.insert(f);
y.insert(a[i]);
y.erase(3);
chmax(dp[i+1][a[i]][c][e][f],dp[i][c][d][e][f]+(int)x.size());
chmax(dp[i+1][c][d][a[i]][e],dp[i][c][d][e][f]+(int)y.size());
}
}
}
}
}
int ans=0;
rep(c,3){
rep(d,3){
rep(e,3){
rep(f,3){
chmax(ans,dp[n][c][d][e][f]);
}
}
}
}
cout<<ans<<endl;
return 0;
}
