#include <stdio.h>
#define NM 10001

int n;
char c[NM];
int f[30];
int sol;
int i, j;

int main()
{
    freopen("litere.in", "r", stdin);
    freopen("litere.out", "w", stdout);
    scanf("%d", &n);
    scanf("%c", &c[0]);
    scanf("%s", &c);
    for ( i = n; i >= 1; i-- )
    {
        j = (int)(c[i-1]-'a'+1);
        for ( int h = 1; h < j; h++ )
            sol += f[h];
        f[j]++;
    }
    printf("%d\n", sol);    
    
    return 0;
}