#include <cstdio>

using namespace std;

int a[26], i, nr, n;
char c;

int main(){
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&n);
    for(i=1;i<=n;i++){
        scanf("%c",&c);
        a[c-'a']++;
        for (int j=c-'a'+1;j<26;j++) nr += a[j];
    }
    printf("%d\n",nr);
    return 0;

}