#include<bits/stdc++.h>
using namespace std;
int n,i,j,ans,x[100005],p,q;
string s;
array<int,3> a,b;
int give(int a,int b,int c){
int ans=0;
if(a==1 || b==1 || c==1) ans++;
if(a==2 || b==2 || c==2) ans++;
if(a==3 || b==3 || c==3) ans++;
return ans;
}
array<int,3> split(array<int,3> a,int val){
return {val,a[0],a[1]};
}
int solve(){
int i,j,k,ans=0;
for(i=0;i<(1<<(n-1));i++){
int cnt=0;
int a1=0,a2=0,a3=0,b1=0,b2=0,b3=0;
for(j=0;j<n;j++){
if(i&(1<<j)){
a3=a2;
a2=a1;
a1=x[j];
for(k=1;k<=3;k++){
if(a1==k || a2==k || a3==k) cnt++;
}
}
else{
b3=b2;
b2=b1;
b1=x[j];
for(k=1;k<=3;k++){
if(b1==k || b2==k || b3==k) cnt++;
}
}
}
ans=max(ans,cnt);
}
return ans;
}
int main(){
cin >> n >> s;
for(i=0;i<n;i++){
if(s[i]=='M') x[i]=1;
if(s[i]=='F') x[i]=2;
if(s[i]=='B') x[i]=3;
}
if(n<=20){
cout << solve();
return 0;
}
for(i=0;i<n;i++){
if(a[0]==b[0] && a[1]==b[1]){
a=split(a,x[i]);
ans+=give(a[0],a[1],a[2]);
continue;
}
p=give(x[i],a[0],a[1]);
q=give(x[i],b[0],b[1]);
if(p>q){
a=split(a,x[i]);
ans+=p;
continue;
}
if(q>p){
b=split(b,x[i]);
ans+=q;
continue;
}
if(p==1){
ans++;
if(a[0]==1 && b[0]==1){
if(a[1]==0) a=split(a,x[i]);
else b=split(b,x[i]);
}
else{
if(a[0]==0) a=split(a,x[i]);
else b=split(b,x[i]);
}
continue;
}
if(p==2){
ans+=2;
if(a[0]==b[0] || a[1]==b[1]) a=split(a,x[i]);
else{
if(a[0]==x[i]) b=split(b,x[i]);
else a=split(a,x[i]);
}
continue;
}
ans+=3;
a=split(a,x[i]);
}
cout << ans;
}
