#define _CRT_SECURE_NO_WARNINGS
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
const int M = 2e5 + 10;
const ll inf = 1e18;
int dp[M][4][4][4][4];
int a[M];
void solve()
{
int n;
cin >> n;
string s;
cin >> s;
//cout << s << endl;
for (int i = 0; i < n; i++)
{
if (s[i] == 'M')
a[i + 1] = 1;
else if (s[i] == 'F')
a[i + 1] = 2;
else if (s[i] == 'B')
a[i + 1] = 3;
}
//for (int i = 1; i <= n; ++i)
// cout << a[i] << " ";
dp[1][0][a[1]][0][0] = 1;
dp[1][0][0][0][a[1]] = 1;
function<int(int, int, int)> ans = [](int x, int y, int z) -> int {
set<int> s;
s.insert(x), s.insert(y), s.insert(z);
s.erase(0);
return s.size();
};
for (int i = 1; i < n; i++)
{
for (int last1 = 0; last1 <= 3; last1++)
{
for (int last2 = 0; last2 <= 3; last2++)
{
for (int last3 = 0; last3 <= 3; ++last3)
{
for (int last4 = 0; last4 <= 3; ++last4)
{
if (dp[i][last1][last2][last3][last4] == 0)
continue;
dp[i + 1][last1][last2][last4][a[i + 1]] = max(
dp[i + 1][last1][last2][last4][a[i + 1]],
dp[i][last1][last2][last3][last4] +
ans(last3, last4, a[i + 1]));
dp[i + 1][last2][a[i + 1]][last3][last4] = max(
dp[i + 1][last2][a[i + 1]][last3][last4],
dp[i][last1][last2][last3][last4] +
ans(last1, last2, a[i + 1]));
}
}
}
}
}
int answer = 0;
for (int last1 = 0; last1 <= 3; last1++)
{
for (int last2 = 0; last2 <= 3; last2++)
{
for (int last3 = 0; last3 <= 3; ++last3)
{
for (int last4 = 0; last4 <= 3; ++last4)
{
answer = max(answer,
dp[n][last1][last2][last3][last4]);
}
}
}
}
cout << answer << endl;
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
solve();
return 0;
}
