www.pudn.com > zju_1000_1010.rar > 1004.cpp

//761 2003-06-07 14:17:10 Accepted 1004 C++ 00:00:14 380K just for play  
#include 
 
int main() 
{ 
	int n,m; 
	while(scanf("%d %d",&n,&m)!=EOF) 
	{ 
		double res=1; 
		int i,j; 
		double t=1.0; 
		for(i=1;i<=n-m;i++) 
		{ 
		 
			double  f=(i%2)?-1:1; 
			double temp=1; 
			for(j=1;j<=i;j++) 
				temp*=1.0/j; 
			t+=f*temp;	 
		} 
		for(i=1;i<=m;i++) 
			t/=(double)i; 
		printf("%.8lf\n",t); 
	} 
	return 0; 
}