#include <bits/stdc++.h>
#define maxn (int)(1e5+51)
#define all(x) x.begin(), x.end()
#define sz(x) (int) x.size()
#define endl '\n'
#define ll long long
#define pb push_back
#define ull unsigned long long
#define ii pair<int,int>
#define iii tuple<int,int,int>
#define inf 2000000001
#define mod 1000000007 //998244353
#define _ ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
using namespace std;
template<typename X, typename Y> bool ckmin(X& x, const Y& y) { return (y < x) ? (x=y,1):0; }
template<typename X, typename Y> bool ckmax(X& x, const Y& y) { return (x < y) ? (x=y,1):0; }
int n,pos[maxn],dp[maxn][3][3][3][3][3][3];
bool mark[maxn][3][3][3][3][3][3];
string s;
int solve(int i,int x1,int x2,int x3,int x4,int x5,int x6){
if(i==n)return 0;
if(mark[i][x1][x2][x3][x4][x5][x6])return dp[i][x1][x2][x3][x4][x5][x6];
mark[i][x1][x2][x3][x4][x5][x6] = 1;
bool d[3]={0,0,0};
if(x3)d[x1]=1;
if(x3>1)d[x2]=1;
d[pos[i]] = 1;
int delta = d[0]+d[1]+d[2];
ckmax(dp[i][x1][x2][x3][x4][x5][x6],solve(i+1,pos[i],x1,min(x3+1,2),x4,x5,x6)+delta);
d[0] = d[1] = d[2] = delta = 0;
if(x6)d[x4]=1;
if(x6>1)d[x5]=1;
d[pos[i]] = 1;
delta = d[0]+d[1]+d[2];
ckmax(dp[i][x1][x2][x3][x4][x5][x6],solve(i+1,x1,x2,x3,pos[i],x4,min(x6+1,2))+delta);
return dp[i][x1][x2][x3][x4][x5][x6];
}
int main(){_
cin>>n>>s;
for(int i=0;i<n;++i){
if(s[i]=='M')pos[i]=0;
else if(s[i]=='B')pos[i]=1;
else pos[i]=2;
}
cout<<solve(0,0,0,0,0,0,0)<<endl;
}
