#include<stdio.h>

int i,j[10000],max,ok,n;
char a[10003];

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

	scanf("%d",&n);

	scanf("%s",&a);

	max=1;

	for(;!ok;)
	{
		ok=1;

		for(i=0;i<n-1;i++)
			if(a[i]>a[i+1])
			{
				a[i]^=a[i+1];
				a[i+1]^=a[i];
				a[i]^=a[i+1];

				j[1]++;

				if(a[i]<a[i-1]||a[i+1]>a[i+2])
					ok=0;
			}

		for(i=1;j[i]>9;i++)
		{
			j[i+1]+=j[i]/10;
			j[i]%=10;

			if(max<i+1)
				max=i+1;
		}
	}

	for(i=max;i>=1;i--)
		printf("%d",j[i]);

	return 0;
}