#include <iostream>
using namespace std;
int main()
{
int N;
cin >> N;
int a[N+1];
char c;
for(int i = 1; i <= N; i++)
{
cin >> c;
if(c == 'M') a[i] = 1;
else if(c == 'F') a[i] = 2;
else if(c == 'B') a[i] = 3;
}
if(N == 1)
{
cout << "1\n";
return 0;
}
if(N == 2)
{
cout << 2 + (a[1] != a[2]) << '\n';
return 0;
}
int res = N;
//mine 1 new, mine 1 old, mine 2 new, mine 2 old
//0 = blank
int dp_new[4][4][4][4], dp_old[4][4][4][4];
for(int i = 0; i <= 3; i++) for(int j = 0; j <= 3; j++)
for(int k = 0; k <= 3; k++) for(int l = 0; l <= 3; l++)
dp_old[i][j][k][l] = -2000000000;
dp_old[0][0][0][0] = 0;
for(int n = 1; n <= N; n++)
{
//cout << "\n\n\n n = " << n << '\n';
for(int i = 0; i <= 3; i++) for(int j = 0; j <= 3; j++)
for(int k = 0; k <= 3; k++) for(int l = 0; l <= 3; l++)
dp_new[i][j][k][l] = -2000000000;
for(int j = 0; j <= 3; j++)
for(int k = 0; k <= 3; k++) for(int l = 0; l <= 3; l++)
{
//i, j, k, l belong to three options, {0, a[n] and !a[n]}
//Insert code below
//i is assumed to be a[n], and can be swapped with k
if(j == 0) dp_new[a[n]][0][k][l] = dp_new[k][l][a[n]][0] = dp_old[0][0][k][l] + 1;
else if(j == a[n])
{
for(int x = 0; x <= 3; x++)
dp_new[a[n]][a[n]][k][l] = dp_new[k][l][a[n]][a[n]] = max(dp_new[a[n]][a[n]][k][l], dp_old[a[n]][x][k][l] + 1 + (x != a[n] && x != 0));
}
else
{
for(int x = 0; x <= 3; x++)
dp_new[a[n]][j][k][l] = dp_new[k][l][a[n]][j] = max(dp_new[a[n]][j][k][l], dp_old[j][x][k][l] + 2 + (x != a[n] && x != j && x != 0));
}
if(n == N) res = max(res, dp_new[a[n]][j][k][l]);
//cout << j << ' ' << k << ' ' << l << " -> " << dp_new[a[n]][j][k][l] << '\n';
}
swap(dp_new, dp_old);
}
cout << res << '\n';
}
