#include <iostream>
using namespace std;
int dp[100001][4][4][4][4];
int frv[256];
int ce_punem(int a,int b,int c){
if(!a) a=c;
if(!b) b=c;
if(a!=b && a!=c && b!=c) return 3;
if(a==b && b==c) return 1;
return 2;
}
int main()
{
frv['M']=1;frv['B']=2;frv['F']=3;
int n,i,food11,food21,food12,food22,max1=0;string s;
cin>>n>>s;
s=' '+s;
for(i=0;i<=n;i++){
for(food11=0;food11<4;food11++){
for(food21=0;food21<4;food21++){
for(food12=0;food12<4;food12++){
for(food22=0;food22<4;food22++){
dp[i][food11][food21][food12][food22]=-1e9;
}
}
}
}
}
dp[0][0][0][0][0]=0;
for(i=1;i<=n;i++){
for(food11=0;food11<4;food11++){///1
for(food21=0;food21<4;food21++){///1
for(food12=0;food12<4;food12++){///2
for(food22=0;food22<4;food22++){///2
///punem la 1
dp[i][food21][frv[s[i]]][food12][food22]=max(dp[i][food21][frv[s[i]]][food12][food22],
dp[i-1][food11][food21][food12][food22]+ce_punem(food11,food12,frv[s[i]]));
dp[i][food11][food21][food22][frv[s[i]]]=max(dp[i][food11][food21][food22][frv[s[i]]],
dp[i-1][food11][food21][food12][food22]+ce_punem(food12,food22,frv[s[i]]));
}
}
}
}
}
/*
for(i=1;i<=n;i++){
for(food11=0;food11<4;food11++){///1
for(food21=0;food21<4;food21++){///1
for(food12=0;food12<4;food12++){///2
for(food22=0;food22<4;food22++){///2
if(dp[i][food11][food21][food12][food22]>0)
cout<<i<<"|1: "<<food11<<" "<<food21<<"|2: "<<food12<<" "<<food22<<" "<<dp[i][food11][food21][food12][food22]<<"\n";
}
}
}
}
}*/
for(food11=0;food11<4;food11++){///1
for(food21=0;food21<4;food21++){///1
for(food12=0;food12<4;food12++){///2
for(food22=0;food22<4;food22++){///2
max1=max(max1,dp[n][food11][food21][food12][food22]);
}
}
}
}
cout<<max1-1;
return 0;
}
