#include<cstdio>
#include<string.h>
using namespace std;
int n;
char s[10000];
void ReadData(){
	int i ; 
	freopen("litere.in","r",stdin);
	scanf("%d",&n);
	scanf("%c",&s[0]);
	for(i=0;i<n;++i){
		scanf("%c",&s[i]);
	}
}
int Solve(){
	int nr = 0,j,i;
	for(i=0;i<n;++i)
		for(j=i+1;j<n;++j)
			if(s[i]>s[j])
				++nr;
	return nr;
}
int main(){
	ReadData();
	freopen("litere.out","w",stdout);
	printf("%d\n",Solve());
	return 0;
}