#include <bits/stdc++.h>
#define ll long long
using namespace std;
#define f first
#define s second
#define pb push_back
#define ep emplace
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define uniquev(v) sort(all(v)), (v).resize(unique(all(v)) - (v).begin())
#define mem(f,x) memset(f , x , sizeof(f))
#define sz(x) (int)(x).size()
#define __lcm(a, b) (1ll * ((a) / __gcd((a), (b))) * (b))
#define mxx *max_element
#define mnn *min_element
#define cntbit(x) __builtin_popcountll(x)
#define len(x) (int)(x.length())
const int N = 1e5 + 10;
int type[N];
int dp[N][16][16], id[16][16];
pair <int, int> f[16];
void maximize(int& a, int b) {
if (a < b)
a = b;
}
int cal(pair <int, int> a, int b) {
vector <bool> used(4, 0);
used[a.f] = 1;
used[a.s] = 1;
used[b] = 1;
int ans = 0;
for (int j = 1; j < 4; j++)
ans += used[j];
return ans;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n;
cin >> n;
int cnt = 0;
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
id[i][j] = cnt;
f[cnt] = {i, j};
cnt++;
}
}
for (int i = 1; i <= n + 1; i++) {
for (int j = 0; j < 16; j++) {
for (int k = 0; k < 16; k++) {
dp[i][j][k] = -1e8;
}
}
}
for (int i = 1; i <= n; i++) {
char c;
cin >> c;
if (c == 'M') {
type[i] = 1;
}
if (c == 'F') {
type[i] = 2;
}
if (c == 'B') {
type[i] = 3;
}
}
dp[1][0][0] = 0;
int ans = 0;
for (int i = 1; i <= n; i++) {
for (int j = 0; j < 16; j++) {
for (int k = 0; k < 16; k++) {
pair <int, int> A = f[j], B = f[k];
int nxt = type[i];
maximize(dp[i + 1][id[A.s][nxt]][k], dp[i][j][k] + cal(A, nxt));
maximize(dp[i + 1][j][id[B.s][nxt]], dp[i][j][k] + cal(B, nxt));
}
}
}
for (int i = 0; i < 16; i++) {
for (int j = 0; j < 16; j++)
maximize(ans, dp[n + 1][i][j]);
}
cout << ans << '\n';
return 0;
}
