// #include "aliens.h"
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define MASK(i) (1LL << (i))
#define GETBIT(mask, i) (((mask) >> (i)) & 1)
#define ALL(v) (v).begin(), (v).end()
#define block_of_code if(true)
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll gcd(ll a, ll b){return __gcd(a, b);}
ll lcm(ll a, ll b){return a / gcd(a, b) * b;}
ll LASTBIT(ll mask){return (mask) & (-mask);}
int pop_cnt(ll mask){return __builtin_popcountll(mask);}
int ctz(ull mask){return __builtin_ctzll(mask);}
int logOf(ull mask){return 63 - __builtin_clzll(mask);}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rngesus(ll l, ll r){return l + (ull) rng() % (r - l + 1);}
double rngesus_d(double l, double r){
double cur = rngesus(0, MASK(60) - 1);
cur /= MASK(60) - 1;
return l + cur * (r - l);
}
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b) {a = b; return true;}
return false;
}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b) {a = b; return true;}
return false;
}
template <class T>
void printArr(T container, string separator = " ", string finish = "\n", ostream &out = cout){
for(auto item: container) out << item << separator;
out << finish;
}
template <class T>
void remove_dup(vector<T> &a){
sort(ALL(a));
a.resize(unique(ALL(a)) - a.begin());
}
const int N = 1e5 + 69;
int nxt[4 * 4][3];
int cnt[4 * 4][3];
int dp[2][4 * 4][4 * 4];
int main(void){
ios::sync_with_stdio(0);cin.tie(0); cout.tie(0);
clock_t start = clock();
for(int i = 0; i<=3; ++i) for(int j = 0; j <= 3; ++j){
int cur = i * 4 + j;
if (i == 0 && j > 0) continue;
for(int k = 1; k <= 3; ++k){
int lext, cu_cnt;
if (i == 0){
lext = k * 4;
cu_cnt = 1;
}
else if (j == 0){
lext = cur + k;
cu_cnt = 1 + (i != k);
}
else{
lext = j * 4 + k;
vector<int> v = {i, j, k};
remove_dup(v);
cu_cnt = v.size();
}
nxt[cur][k-1] = lext;
cnt[cur][k-1] = cu_cnt;
}
}
memset(dp, -1, sizeof dp);
dp[0][0][0] = 0;
int n; cin >> n;
string s; cin >> s;
int ans = 0;
for(char c: s){
int digit;
if (c == 'M') digit = 0;
else if (c == 'F') digit = 1;
else digit = 2;
for(int j = 0; j < 4*4; ++j) for(int k = 0; k < 4*4; ++k) if (dp[0][j][k] != -1){
maximize(dp[1][nxt[j][digit]][k], dp[0][j][k] + cnt[j][digit]);
maximize(dp[1][j][nxt[k][digit]], dp[0][j][k] + cnt[k][digit]);
}
for(int j = 0; j < 4*4; ++j) for(int k = 0; k < 4*4; ++k){
dp[0][j][k] = dp[1][j][k];
dp[1][j][k] = -1;
maximize(ans, dp[0][j][k]);
}
}
cout << ans << "\n";
cerr << "Time elapsed: " << clock() - start << " ms\n";
return 0;
}
