#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;
}
const int N=100100;
const int K=8;
int dp[N][1 << K];
int get_bit(int v,int k) {
return (v >> k) & 1;
}
int eval(int v,int k,int n) {
// cout << bitset<8>(v) << " and " << k << " becomes ";
if (k)
v = v >> 4;
// cout << bitset<4>(v) << endl;
int mask = (1 << (get_bit(v,0) + 2 * get_bit(v,1))) | (1 << (get_bit(v,2) + 2 * get_bit(v,3))) | (1 << n);
return get_bit(mask,1) + get_bit(mask,2) + get_bit(mask,3);
}
int bts[8];
int insert(int v,int k,int n) {
for (int i=0;i<8;i++)
bts[i]=get_bit(v,i);
if (k) {
bts[4]=bts[6],bts[5]=bts[7],bts[6]=get_bit(n,0),bts[7]=get_bit(n,1);
int res=0;
for (int i=0;i<8;i++)
res|=bts[i]*(1<<i);
return res;
} else {
bts[4-4]=bts[6-4],bts[5-4]=bts[7-4],bts[6-4]=get_bit(n,0),bts[7-4]=get_bit(n,1);
int res=0;
for (int i=0;i<8;i++)
res|=bts[i]*(1<<i);
return res;
}
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
string s;
cin >> s;
s = "#" + s;
string wrs="BFM";
memset(dp,0xc0,sizeof(dp));
dp[0][0]=0;
for (int i=1;i<=n;i++) {
int v = find(wrs.begin(),wrs.end(),s[i])-wrs.begin() + 1;
for (int prev_state=0;prev_state<(1<<K);prev_state++) {
for (int k=0;k<2;k++) {
auto new_state = insert(prev_state,k,v);
// cout << bitset<8>(prev_state) << " " << k << " " << v << " " << bitset<8>(new_state) << " " << eval(prev_state,k,v) << endl;
cmax(dp[i][new_state],dp[i-1][prev_state] + eval(prev_state,k,v));
}
}
}
int ans=0;
for (int i=0;i<(1<<K);i++)
cmax(ans,dp[n][i]);
cout << ans << endl;
}
