ll n;string s;
ll f(ll t){
	ll a=0,b=0;
	rep(i,n){
		if(s[i]=='A'||s[i]=='?'&&i<t){
			a+=1;
		}else{
			b+=a;
		}
	}
	return b;
}
{
	rd(n,s);
	wt(tsearch_max[ll,t,0,n+1](f(t)));
}