#include<cstdio>
#define MAXN 10010
#define SIGMA 30
using namespace std;
char s[MAXN];
int frequency[SIGMA];
int main(){
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    int n,answer=0,i,j,letter;
    scanf("%d\n%s",&n,s+1);
    for(i=1;i<=n;i++){
        letter=s[i]-97;
        frequency[letter]++;
        for(j=letter+1;j<=26;j++)
            answer+=frequency[j];
    }
    printf("%d",answer);
    return 0;
}