E求条（有效悬5关）

string s;
int n;
ll as[10000000],bs[10000000],c[10000000],t[10000000],ans=0;
void msort(int l,int r){
	if(l==r){
		return;
	}
	int mid=(l+r)/2;
	int i=l,j=mid+1,k=l;
	msort(l,mid);
	msort(mid+1,r);
	while(i<=mid&&j<=r){
		if(c[i-1]>=c[j]){
			t[k++]=c[i++];
		}else{
			t[k++]=c[j++];
			ans+=mid-i+1;
		}
	}
	while(i<=mid){
		t[k++]=c[i++];
	}
	while(j<=r){
		t[k++]=c[j++];
	}
	for(int i=l;i<=r;i++){
		c[i]=t[i];
	}
}
int main(){
	fastrd
	//struct _timeb T;
	//_ftime(&T);
	//srand(T.millitm);
	cin>>n>>s;
	for(int i=0;i<n;i++){
		as[i+1]=as[i]+(s[i]=='A'?1:0);
		bs[i+1]=bs[i]+(s[i]=='B'?1:0);
		c[i+1]=as[i+1]-bs[i+1];
	}
	
	msort(1,n);
	cout<<ans;
}