#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template<class T> bool cmin(T &i, T j) { return i > j ? i=j,true:false; }
template<class T> bool cmax(T &i, T j) { return i < j ? i=j,true:false; }
mt19937 mrand(chrono::steady_clock::now().time_since_epoch().count());
uniform_int_distribution<int> ui(0, 1 << 30);
int rng() {
return ui(mrand);
}
struct state {
int a[3][2];
bool operator==(const state &e) {
for (int i=0;i<3;i++)
for (int j=0;j<2;j++)
if (a[i][j]!=e.a[i][j])
return 0;
return 1;
}
};
state shift(state &p,int w,int n) {
state res = p;
res.a[0][w]=res.a[1][w];
res.a[1][w]=res.a[2][w];
res.a[2][w]=n;
return res;
}
int eval(state &s,int w) {
int mask = 0;
for (int i=0;i<3;i++)
mask |= 1 << s.a[i][w];
int ans = 0;
for (int i=1;i<=3;i++)
ans += (mask >> i) & 1;
return ans;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
string s;
cin >> s;
s = "#" + s;
string wrs="BFM";
vector<vector<pair<state,int>>> dp(n + 1);
state def; memset(def.a,0,sizeof(def.a));
dp[0].emplace_back(def,0);
for (int i=1;i<=n;i++) {
int v = find(wrs.begin(),wrs.end(),s[i])-wrs.begin() + 1;
for (auto &[prev_state,val]: dp[i-1]) {
for (int k=0;k<2;k++) {
auto new_state = shift(prev_state,k,v);
bool fnd=0;
for (auto &[now,cval]: dp[i]) if (new_state == now) {
cmax(cval,val + eval(new_state,k));
fnd=1;
break;
}
if (!fnd) {
dp[i].emplace_back(new_state,val + eval(new_state,k));
}
}
}
}
int ans = 0;
for (auto &[s,val]: dp[n])
cmax(ans,val);
cout << ans << endl;
}
