gistfile1.hpp

#include <stdio.h>
#include <string.h>
#include <stdlib.h>


int main() {
	int i,j,N,K,cas=1;
	double ans,tmp;
	
	
//	freopen("test.in", "r", stdin);
//	freopen("test.out", "w", stdout);
	while(scanf("%d%d", &N, &K) != EOF) {
		if(N == 0 && K == 0)
			break;
		if(N == 1)
			printf("Case %d: %.4f\n", cas++, 0.0);
		else if(K * 2 + 1 >= N)
			printf("Case %d: %.4f\n", cas++, (double)N);
		else {
			ans = 0.0;
			for(i=N-1,j=N-(K*2+1),tmp=1.0; i>=N-K*2; i--,j--) {
				tmp *= (double)j / i;
			}
			ans += (1.0 - tmp) * (N - K*2);
	//		printf("%f %f\n", tmp, ans);
			for(i=N-K*2; i<=N-(K+1); i++) {
				if(i - K*2 == 0) {
					for(i=1,tmp=1.0; i<=K*2; i++) {
						tmp *= (double)i / (N - K*2 + i - 1);
					}
					i--;
					if(i >= N - K*2 && i <= N - (K + 1))
						ans += (1.0 - tmp) * 2.0;
				}
				else {
					tmp *= (double)i / (i - K*2);
					ans += (1.0 - tmp) * 2.0;
				}
	//			printf("%f %f\n", tmp, ans);
			}
			printf("Case %d: %.4f\n", cas++, ans);		
		}
	}
	return 0;
}
				

Judge verdict: WA

If you use the long double instead double, your code will get Accepted!!