#include <cstdio>

using namespace std;

char s[10005];
int v[30];

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

    int n, ans=0;

    scanf( "%d\n%s", &n, &s );

    for( int i=0;i<n;i++ )
    {
        v[s[i]-'a']++;

        for( int j=(s[i]-'a')+1;j<=25;j++ )
            ans+=v[j];
    }

    printf( "%d", ans );

    return 0;
}