#include <iostream>
#include <unordered_map>
#include <cstring>
using namespace std;
const int MAXN = 100000;
char shipment[MAXN + 2];
int N;
int memo[MAXN][4][4][4][4];
struct Prev {
char a[2][2];
};
Prev give(int pos, int mine, Prev prev) {
prev.a[mine][0] = prev.a[mine][1];
prev.a[mine][1] = shipment[pos];
return prev;
}
int diff(int pos, int mine, Prev prev) {
unordered_map<char, int> foodCount;
foodCount[prev.a[mine][0]]++;
foodCount[prev.a[mine][1]]++;
foodCount[shipment[pos]]++;
int typesOfFood = 0;
if (foodCount['M'])
typesOfFood++;
if (foodCount['F'])
typesOfFood++;
if (foodCount['B'])
typesOfFood++;
return typesOfFood;
}
unordered_map<char, int> conversion;
int F(int pos, Prev prev) {
if (pos == N) {
return 0;
} else {
int &m = memo[pos][ conversion[prev.a[0][0]] ][ conversion[prev.a[0][1]] ][ conversion[prev.a[1][0]] ][ conversion[prev.a[1][1]] ];
if (m==-1)
m = max(
F(pos + 1, give(pos, 0, prev)) + diff(pos, 0, prev),
F(pos + 1, give(pos, 1, prev)) + diff(pos, 1, prev)
);
return m;
}
}
int main() {
memset(memo,-1,sizeof memo);
conversion[0] = 0;
conversion['M'] = 1;
conversion['F'] = 2;
conversion['B'] = 3;
cin >> N >> shipment;
cout << F(0, {{{0, 0}, {0, 0}}}) << '\n';
}
