Red is good

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<iostream>
 4 #define dd double
 5 using namespace std;
 6 
 7 int R,B,now;
 8 dd f[2][5066];
 9 
10 int main(){
11     
12     scanf("%d%d",&R,&B);
13     
14     now=0;
15     dd temp;
16     for(int i=1;i<=R;++i)
17     {
18         now^=1;
19         f[now][0]=i;
20         for(int j=1;j<=B;++j)
21         {
22             temp=(f[now][j-1]-1)*(dd)(j)/(dd)(i+j)+(f[now^1][j]+1)*(dd)(i)/(dd)(i+j);
23           f[now][j]=temp>0?temp:0;
24         }
25     }
26     printf("%.6lf",f[now][B]-0.0000005);
27     
28     //while(1);
29 }