cf 148D Bag of mice

The dragon 选一只老鼠，然后会跑掉一只

the princess选一只老鼠，不会跑出另外的老鼠

求the princess赢的概率

#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<queue>
#include<stack>
#include<algorithm>
#include<cstring>
#include<stdlib.h>
#include<cmath>
using namespace std;
#define pb push_back
double dp[1010][1010];
int main(){
    int w,b;
    while(cin>>w>>b){
        memset(dp,0,sizeof(dp));
        dp[1][0]=1;
        for(int i=1;i<=w;i++)
        for(int j=0;j<=b;j++){
            double p1,p2;
            int a=i,b=j;
            dp[i][j]=1.0*i/(i+j);
            if(b>=1){
                p1=1.0*b/(a+b);
                b--; //the princess选了一只
                if(b>=1){
                    p2=1.0*b/(a+b);//The dragon 选一只
                    b--;
                    if(b>=1)
                        dp[i][j]+=p1*p2*b/(a+b)*dp[i][j-3];
                    dp[i][j]+=p1*p2*a/(a+b)*dp[i-1][j-2];
                }
            }
        }
        printf("%.10lf
",dp[w][b]);
    }
}