#include<cstdio>
#define maxn 100005

using namespace std;

char s[maxn];
int n,i,j,k,a[170];

int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);

    scanf("%d\n",&n);
    fgets(s,n+2,stdin);

    for(i=0;i<n;++i)
    if(s[i]>='a'&&s[i]<='z')
    {
        for(j=s[i]+1;j<='z';++j)
            k+=a[j];
        ++a[s[i]];
    }

    printf("%d\n",k);
    fclose(stdin);
    fclose(stdout);

    return 0;
}