#include <bits/stdc++.h>
using namespace std;
ifstream in("litere.in");
ofstream out("litere.out");
int n,su,f[1000];
char s;
int main()
{
    in>>n;
    for(int i=1;i<=n;i++)
    {
        in>>s;
        for(char p=s+1;p<'z';p++)
         su+=f[p];
        f[s]++;
    }
    out<<su;
    return 0;
}
///1 2 1 3 3 2 1 1 3
///1 1 2 1 2 2 3 4 3