#include <cstdio>
#include <algorithm>

using namespace std;

char b[100050];

int main(){
	int n,i,j=0,cur=0,q;
	
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	
	scanf("%d %s", &n, b);
	
	for(i=0;i<n;++i){
		for(q=i+1;q<n;++q){
			if(b[i]>b[q])
				cur++;
	    }
	}
	printf("%d", cur);

	return 0;
}