#include<fstream>
#include<algorithm>
#include<vector>
#include<cstring>
//#include<iostream>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
char s[10005];int v[33];
int main() {
    int n,cnt=0;
    cin>>n;
    cin>>s;
    n--;
    for(int i=0;i<=n;i++)
    {
        int x=s[i]-97;
        v[x]++;
        for(int j=x+1;j<='z'-97;j++)
            cnt+=v[j];
    }
    cout<<cnt;

}