#include <iostream>
#include <algorithm>
const int MAXN = 1e5 + 1;
const int NOPE = -1e9;
int dp[MAXN][4][4][4][4];
int main()
{
std::ios::sync_with_stdio(false);
std::cout.tie(nullptr);
std::cin.tie(nullptr);
int shipments; std::cin >> shipments;
std::fill_n((int *)dp, (shipments + 1) * 256, NOPE);
dp[0][0][0][0][0] = 0;
for (int iShipment = 0; iShipment < shipments; iShipment++)
{
char t; std::cin >> t;
int type = 0;
switch (t)
{
case 'M':
type = 1;
break;
case 'F':
type = 2;
break;
case 'B':
type = 3;
break;
}
for (int a = 0; a < 4; a++)
for (int b = 0; b < 4; b++)
{
int nxt1 = 0;
if (a == 0 && b == 0)
nxt1 = 1;
else if (a == 0)
nxt1 = 2 - (b == type);
else
nxt1 = 3 - std::min(2, (a == b) + (b == type) + (a == type));
for (int c = 0; c < 4; c++)
for (int d = 0; d < 4; d++)
{
if (dp[iShipment][a][b][c][d] == NOPE)
continue;
int nxt2 = 0;
if (c == 0 && d == 0)
nxt2 = 1;
else if (c == 0)
nxt2 = 2 - (d == type);
else
nxt2 = 3 - std::min(2, (c == d) + (d == type) + (c == type));
dp[iShipment + 1][b][type][c][d] = std::max(dp[iShipment + 1][b][type][c][d], dp[iShipment][a][b][c][d] + nxt1);
dp[iShipment + 1][a][b][d][type] = std::max(dp[iShipment + 1][a][b][d][type], dp[iShipment][a][b][c][d] + nxt2);
}
}
}
int ans = 0;
for (int a = 0; a < 4; a++)
for (int b = 0; b < 4; b++)
for (int c = 0; c < 4; c++)
for (int d = 0; d < 4; d++)
ans = std::max(ans, dp[shipments][a][b][c][d]);
std::cout << ans << '\n';
return 0;
}
