codeforces 148D 概率DP

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <iostream>
#include <iomanip>
#include <cmath>
#include <map>
#include <set>
#include <queue>
using namespace std;

#define ls(rt) rt*2
#define rs(rt) rt*2+1
#define ll long long
#define ull unsigned long long
#define rep(i,s,e) for(int i=s;i<e;i++)
#define repe(i,s,e) for(int i=s;i<=e;i++)
#define CL(a,b) memset(a,b,sizeof(a))
#define IN(s) freopen(s,"r",stdin)
#define OUT(s) freopen(s,"w",stdout)
const ll ll_INF = ((ull)(-1))>>1;
const double EPS = 1e-8;
const int INF = 100000000;

const int MAXN = 1000+100;
double dp[MAXN][MAXN];

int main()
{
    int w,b;
    while(~scanf("%d%d",&w,&b))
    {
        CL(dp,0);
        for(int i=1;i<=w;i++)dp[i][0]=1.0;
        for(int i=1;i<=b;i++)dp[0][i]=0.0;
        for(int i=1;i<=w;i++)
            for(int j=1;j<=b;j++)
            {
                dp[i][j]=1.0*i/(i+j);
                if(j>=3)dp[i][j]+=1.0*j*(j-1)*(j-2)/(1.0*(i+j)*(i+j-1)*(i+j-2))*dp[i][j-3];
                if(i>=1 && j>=2)dp[i][j]+=1.0*i*j*(j-1)/(1.0*(i+j)*(i+j-1)*(i+j-2))*dp[i-1][j-2];
            }
        printf("%.9lf\n",dp[w][b]);
    }
    return 0;
}