#include <fstream>
#define nmax 10005
using namespace std;
int n,m,sol;
char s[nmax],t[nmax];
ifstream f("litere.in");
ofstream g("litere.out");

int main()
{
    f>>n>>s;
    int i,j,k;
    for (i=0;i<26;i++) {
        for (j=0,k=0;j<n;j++) {
            if (s[j]==i+'a')
                sol+=j-k++;
        }
        for (j=0,m=0;j<n;j++)
            if (s[j]!=i+'a')
                t[m++]=s[j];
        for (j=0;j<n;j++) {
            s[j]=t[j];
            t[j]=0;
        }
    }
    g<<sol;
}