BZOJ3398 牡牛和牝牛

隔板搞掉就行了

特么我什么时候感冒能好！QAQ。脑子都没有了QAQ。

//Love and Freedom.
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define inf 20021225
#define ll long long
#define mdn 5000011
using namespace std;
int fac[100010],inv[100010];
int ksm(int bs,int mi)
{
	int ans = 1;
	while(mi)
	{
		if(mi&1)	ans=(ll)ans*bs%mdn;
		bs=(ll)bs*bs%mdn; mi>>=1;
	}
	return ans;
}

int C(int x,int y)
{
	if(x<y || y<0)	return 0;
	return (ll)fac[x] * inv[y] %mdn *inv[x-y] %mdn;
}

int main()
{
	int n,k;
	scanf("%d%d",&n,&k); fac[0] = inv[0] = 1;
	for(int i=1;i<=n;i++)	fac[i] = (ll)fac[i-1]*i%mdn;
	inv[n] = ksm(fac[n],mdn-2);// printf("%d\n",fac[n]);
	for(int i=n;i;i--)	inv[i-1]=(ll)inv[i]*i%mdn;
	int ans=1;
	for(int x=1;x<=n;x++)
	{
		ans+=C(n-(x-1)*k,x),ans%=mdn;
		//printf("%d %d %d\n",n-(x-1)*k,x,C(n-(x-1)*k,x));
	}
	printf("%d\n",ans);
	return 0;
}