#include<bits/stdc++.h>
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
int n,v[124],i,j;
string s;
int main()
{
    for(f>>n>>s;i<n;++i)
        for(++v[s[i]],j=122;j>s[i];--j)
            v[0]+=v[j];
    g<<v[0];
    return 0;
}