include<bits/stdc++.h>

using namespace std; long long n,a,b,ans; string str; int main() {

cin>>n>>str;
for(int i=0;i<n;i++)
{
	if(str[i]=='N') a++;
	else if(str[i]=='O') b+=a;
	else ans+=b;
}
cout<<ans;

}