#include <cstdio>


int r[27];
int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    int n;
    scanf("%d\n",&n);
    char c;
    long long rez=0;
    for(int i=1;i<=n;i++)
    {
        scanf("%c",&c);
        c=c-'a'+1;
        rez+=r[c+1];
        for(int j=c;j>0;j--)
            r[j]++;
    }

    printf("%lld",rez);
    return 0;
}