#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")
#pragma GCC optimize("Ofast")
#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
#include <string>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <queue>
#include <deque>
#include <list>
#include <iomanip>
#include <stdlib.h>
#include <time.h>
#include <cstring>
using namespace std;
typedef int ll;
typedef unsigned long long int ull;
typedef long double ld;
#define REP(i,a,b) for(ll i=(ll) a; i<(ll) b; i++)
#define pb push_back
#define mp make_pair
#define pl pair<ll,ll>
#define ff first
#define ss second
#define whole(x) x.begin(),x.end()
#define DEBUG(i) cout<<"Pedro "<<i<<endl
#define INF 100000000
#define EPS ((ld)0.00000000001)
#define pi ((ld)3.141592653589793)
#define VV(vvvv,NNNN,xxxx); REP(iiiii,0,NNNN) {vvvv.pb(xxxx);}
ll mod=1000000007;
template<class A=ll>
void Out(vector<A> a) {REP(i,0,a.size()) {cout<<a[i]<<" ";} cout<<endl;}
template<class A=ll>
void In(vector<A> &a, ll N) {A cur; REP(i,0,N) {cin>>cur; a.pb(cur);}}
vector<string> states; ll S; string s;
vector<vector<ll> > curstate, newstate;
unordered_map<string,ll> m;
ll num_dif(string str)
{
set<char> dif;
REP(i,0,str.size()) {dif.insert(str[i]);}
return dif.size();
}
void update(ll a,ll b,ll c, char added)
{
if(c==1) {swap(a,b);}
ll extra = num_dif(states[a]+added);
string newa = states[a]+added; if(newa.size()==3) {newa.erase(newa.begin());}
ll upd_a = m[newa]; ll upd_b = b;
if(c==1) {swap(upd_a,upd_b);}
newstate[upd_a][upd_b]=max(newstate[upd_a][upd_b],curstate[a][b]+extra);
newstate[upd_b][upd_a]=newstate[upd_a][upd_b];
//cout<<"("<<states[a]<<","<<states[b]<<"),("<<states[upd_a]<<","<<states[upd_b]<<"): "<<added<<" "<<c<<" "<<extra<<endl;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cout.precision(20);
ll N; cin>>N; cin>>s;
states = {"","M","F","B","MM","MF","MB","FM","FF","FB","BM","BF","BB"}; S = states.size();
REP(i,0,S) {m[states[i]]=i;}
curstate = vector<vector<ll> >(S,vector<ll>(S,-INF)); newstate = vector<vector<ll> >(S,vector<ll>(S,-INF));
curstate[0][0]=0;
REP(i,0,N)
{
REP(a,0,S) {REP(b,0,S) {newstate[a][b]=-INF;}}
REP(a,0,S)
{
REP(b,0,a+1)
{
if(curstate[a][b]<0) {continue;}
REP(c,0,2)
{
update(a,b,c,s[i]);
}
}
}
REP(a,0,S) {REP(b,0,S) {curstate[a][b]=newstate[a][b];}}
}
ll ans = 0;
REP(a,0,S) {REP(b,0,a+1) {ans=max(ans,curstate[a][b]);}}
cout<<ans<<endl;
return 0;
}
