#include <stdio.h>
#include <string.h>
double arr[1002][101];
int main(){
	int c,m,n,i,j;
	while (scanf("%d",&c))
	{
		if (!c)		break;
		scanf("%d%d",&n,&m);
		memset(arr,0,sizeof(arr));
		
		if (m>c||m>n){	printf("0.000\n");	continue;	}
		
		if (n>1000)
		{
			if (n%2)	n = 1001;
			else		n = 1000;
		}
		
		arr[1][1] = 1;
		for (i=2;i<=n;++i)
		{
			for (j=0;j<=c;++j)
			{
				if (!j)
					arr[i][j] = arr[i-1][1]/c;
				else if (j==c)
					arr[i][j] = arr[i-1][c-1]/c;
				else arr[i][j] = arr[i-1][j-1]*(c-j+1)/c + arr[i-1][j+1]*(j+1)/c;
			}
		}
		printf("%.3lf\n",arr[n][m]);
	}
	return 0;
}