BZOJ1419: Red is good

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;
#define maxn 5010
#define mod 1000000

int n,m,ans;
double f[2][maxn];

int main(){
    scanf("%d%d",&n,&m);
    int now=0;
    for (int i=1;i<=n;i++,now=1-now){
        f[now][0]=i;
        for (int j=1;j<=m;j++)
            f[now][j]=max(0.0,(f[now^1][j]+1)*i/(i+j)+(f[now][j-1]-1)*j/(i+j));
    }
    long long ans=floor(f[now^1][m]*mod);
    printf("%lld.%06lld",ans/mod,ans%mod);
    return 0;
}