#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 1;
int dp[2][4][4][4][4], ans = 0;
string s;
int diff(int t1, int t2, int t3){
bool f[4] = {0, 0, 0, 0};
f[t1] = f[t2] = f[t3] = 1;
return f[1] + f[2] + f[3];
}
int main(){
memset(dp, -1, sizeof dp);
dp[0][0][0][0][0] = 0;
int n;
string s;
cin >> n >> s;
s = "@" + s;
for(int i = 1; i <= n; ++i){
int t = 1;
if(s[i] == 'B')t = 2;
if(s[i] == 'F')t = 3;
for(int t0 = 0; t0 < 4; ++t0){
for(int t1 = 0; t1 < 4; ++t1){
for(int T0 = 0; T0 < 4; ++T0){
for(int T1 = 0; T1 < 4; ++T1){
int &dp0 = dp[1][t1][t][T0][T1];
int &dp1 = dp[1][t0][t1][T1][t];
int opt = dp[0][t0][t1][T0][T1];
if(~opt){
dp0 = max(dp0, opt + diff(t0, t1, t));
dp1 = max(dp1, opt + diff(T0, T1, t));
}
}
}
}
}
for(int t0 = 0; t0 < 4; ++t0){
for(int t1 = 0; t1 < 4; ++t1){
for(int T0 = 0; T0 < 4; ++T0){
for(int T1 = 0; T1 < 4; ++T1){
dp[0][t0][t1][T0][T1] = dp[1][t0][t1][T0][T1];
dp[1][t0][t1][T0][T1] = -1;
}
}
}
}
}
for(int t0 = 0; t0 < 4; ++t0){
for(int t1 = 0; t1 < 4; ++t1){
for(int T0 = 0; T0 < 4; ++T0){
for(int T1 = 0; T1 < 4; ++T1){
ans = max(ans, dp[0][t0][t1][T0][T1]);
}
}
}
}
cout << ans << endl;
}
