#include<stdio.h>
#include<string.h>

#define N 10000

int main() {
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	
	char sir[N];
	int n,i,j,cnt=0;
	
	scanf("%d",&n); scanf("%c",&sir[0]);
	
	for(i=1; i<=n; i++) {
		scanf("%c",&sir[i]);
		for(j=1; j<=i; j++)
			if(sir[j]>sir[i])
				++cnt;
	}
	
	printf("%d\n",cnt);
	
	return 0;
}