#include <iostream>
using namespace std;
int const INF = 1e9+7;
int dp[2][4][4][4][4];
int add(int a, int b, int c) {
int freq[4];
freq[0] = freq[1] = freq[2] = freq[3] = 0;
freq[a]++;
freq[b]++;
freq[c]++;
int ans = min(1, freq[1]) + min(1, freq[2]) + min(1, freq[3]);
return ans;
}
int main() {
int n;
cin >> n;
for(int a1 = 0;a1 <= 3;a1++) {
for(int a2 = 0;a2 <= 3;a2++) {
for(int b1 = 0;b1 <= 3;b1++) {
for(int b2 = 0;b2 <= 3;b2++) {
dp[0][a1][a2][b1][b2] = -INF;
dp[1][a1][a2][b1][b2] = -INF;
}
}
}
}
dp[0][0][0][0][0] = 0;
for(int i = 1;i <= n;i++) {
char ch;
int type = 0;
cin >> ch;
if(ch == 'M') {
type = 1;
}else if(ch == 'F') {
type = 2;
}else if(ch == 'B') {
type = 3;
}
for(int a1 = 0;a1 <= 3;a1++) {
for(int a2 = 0;a2 <= 3;a2++) {
for(int b1 = 0;b1 <= 3;b1++) {
for(int b2 = 0;b2 <= 3;b2++) {
dp[1][type][a1][b1][b2] = max(dp[1][type][a1][b1][b2], dp[0][a1][a2][b1][b2] + add(a1, a2, type));
dp[1][a1][a2][type][b1] = max(dp[1][a1][a2][type][b1], dp[0][a1][a2][b1][b2] + add(b1, b2, type));
}
}
}
}
for(int a1 = 0;a1 <= 3;a1++) {
for(int a2 = 0;a2 <= 3;a2++) {
for(int b1 = 0;b1 <= 3;b1++) {
for(int b2 = 0;b2 <= 3;b2++) {
dp[0][a1][a2][b1][b2] = dp[1][a1][a2][b1][b2];
dp[1][a1][a2][b1][b2] = -INF;
}
}
}
}
}
int ans = 0;
for(int a1 = 0;a1 <= 3;a1++) {
for(int a2 = 0;a2 <= 3;a2++) {
for(int b1 = 0;b1 <= 3;b1++) {
for(int b2 = 0;b2 <= 3;b2++) {
ans = max(ans, dp[0][a1][a2][b1][b2]);
}
}
}
}
cout << ans;
return 0;
}
