#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using vl=vector<long long>;
using vvl=vector<vector<long long>>;
using vvvl=vector<vector<vector<long long>>>;
using pl=pair<long long,long long>;
using vpl=vector<pair<long long,long long>>;
#define fi first
#define se second
#define all(x) (x).begin(),(x).end()
#define _overload3(_1,_2,_3,name,...) name
#define _rep(i,n) repi(i,0,n)
#define repi(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#include <atcoder/all>
using namespace atcoder;

long long modpow(long long a, long long n, long long mo){long long res=1;while(n>0){if(n&1){res=res*a%mo;}a=a*a%mo;n>>=1;}return res;}
long long Pow(long long a, long long n){long long res=1;while(n>0){if(n&1){res=res*a;}a=a*a;n>>=1;}return res;}

const ll MOD=998244353;
const ll INF=(1ll<<60);

int main(){
  ll N;
  cin>>N;
  string S;
  cin>>S;
  fenwick_tree<ll> fw(N);
  vl memo;
  vl C;
  rep(i,N){
    if(S[i]!='C'){
      fw.add(i,1);
    }
    else C.pb(i);
    if(S[i]=='?') memo.pb(i);
  }
  ll ans=0;
  reverse(all(memo));
  rep(i,C.size()){
    ans+=fw.sum(0,C[i]);
  }
  ll an=ans;
  rep(i,memo.size()){
    an-=(N-memo[i])-fw.sum(memo[i],N);//Cの数
    fw.add(memo[i],-1);
    an+=fw.sum(0,memo[i]);//Aの数
    ans=max(an,ans);
  }
  cout<<ans<<endl;
}