#include<stdio.h>
#define N 10001
int n,nr,p[N];
char s[N];
void read()
{
	char bn;
	scanf("%d%c%s",&n,&bn,&s);
	//printf("%s",s);
}
void solve()
{
	int i,j;
	p[s[0]-'a']=1;
	for(i=1;i<n;++i)
	{
		p[s[i]-'a']++;
		for(j=s[i]-'a'+1;j<27;j++)
			if(p[j])
				nr+=p[j];
	}
	/*for(i=1;i<n;++i)
		for(j=0;j<=i;++j)
			if(s[j]>s[i])
				nr++;*/
	printf("%d\n",nr);
}
int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	read();
	solve();
	return 0;
}