zoj 1619 Present

#include<iostream>#include<cstdio>using namespace std;int main(){int n,m;    long long arr[21];arr[0]=1;arr[1]=1;for(int i=2;i<=20;i++){long long temp=1;for(int j=2;j<=i;j++)temp*=j;        arr[i]=temp;}while(scanf("%d %d",&n,&m)!=EOF){double ans=1;         if(m<=20) { if(n-m<=20) {      for(int i=1;i<=n-m;i++)         if(i%2==0)        ans+=1.0/arr[i];        else        ans-=1.0/arr[i]; } else { for(int i=1;i<=20;i++)         if(i%2==0)        ans+=1.0/arr[i];        else        ans-=1.0/arr[i]; } ans/=arr[m]; } else {     if(n-m<=20) {      for(int i=1;i<=n-m;i++)         if(i%2==0)        ans+=1.0/arr[i];        else        ans-=1.0/arr[i]; } else { for(int i=1;i<=20;i++)         if(i%2==0)        ans+=1.0/arr[i];        else        ans-=1.0/arr[i]; } ans/=arr[20]; } printf("%.8lfn",ans);}return 0;}