#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define int long long
#define sz(x) (int)(x.size())
using namespace std;
using namespace __gnu_pbds;
typedef
tree<
int,
null_type,
less<int>,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;
int n, k, m;
string seq;
map<char, int> vaut;
map<pair<int, pair<pair<int, int>, pair<int, int>>>, int> memo;
int dp(int empl, pair<int, int> prec1, pair<int, int> prec2) {
if (empl>=n) return 0;
if (memo.count({empl, {prec1, prec2}})) return memo[{empl, {prec1, prec2}}];
int s1=dp(empl+1, {prec1.second, vaut[seq[empl]]}, prec2);
int s2=dp(empl+1, prec1, {prec2.second, vaut[seq[empl]]});
map<int, int> mp1;
mp1[vaut[seq[empl]]]++;
if (prec1.first!=-1) mp1[prec1.first]++;
if (prec1.second!=-1) mp1[prec1.second]++;
map<int, int> mp2;
mp2[vaut[seq[empl]]]++;
if (prec2.first!=-1) mp2[prec2.first]++;
if (prec2.second!=-1) mp2[prec2.second]++;
s1+=mp1.size();
s2+=mp2.size();
return memo[{empl, {prec1, prec2}}]=max(s1, s2);
}
void solve() {
vaut['M']=0, vaut['F']=1, vaut['B']=2;
cin >> n;
cin >> seq;
memo.clear();
cout << dp(0, {-1, -1}, {-1, -1}) << endl;
return;
}
signed main() {
ios::sync_with_stdio(0);
cin.tie(0);
int tt=1;// cin >> tt;
while(tt--) solve();
return 0;
}
