#include <cstdio>
char s[10010];
int fr[27],cate,i,j,n;
int main(){
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&n);
    scanf("%s",s);
    for(i=0;i<n;i++){
        fr[s[i]-'a'+1]++;
        for(j=s[i]-'a'+2;j<=26;j++){
            if(fr[j]!=0)
                cate+=fr[j];
        }
    }
    printf("%d",cate);
    return 0;
}