#include <stdio.h>

#define FIN "litere.in"
#define FOUT "litere.out"

int N, cnt[26]; long long Res;

int main(void)
{
    int i, j;
    char c;

    freopen(FIN, "r", stdin);
    freopen(FOUT, "w", stdout);

    scanf("%d\n", &N);
    for (i = 0; i < N; ++i)
    {
        c = getc(stdin);
        for (j = c-'a'+1; j < 26; ++j)
            Res += cnt[j];
        ++cnt[c-'a'];
    }
    printf("%lld\n", Res);

    return 0;
}