#include <cstdio>
#include <cstring>
using namespace std;

const int NMAX = 10010;

long long Ans; 
int N, Freq[30];
char S[NMAX];

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