%%%%http://blog.csdn.net/actoy/article/details/9366485
（看上去好麻烦的样子啊）
1 #include<cstdio> 2 #include<iostream> 3 #include<algorithm> 4 #include<cstdlib> 5 #define LL long long 6 #define LD long double 7 #define N 100005 8 using namespace std; 9 inline int ra() 10 { 11 int x=0,f=1; char ch=getchar(); 12 while (ch<'0' || ch>'9') {if (ch=='-') f=-1; ch=getchar();} 13 while (ch>='0' && ch<='9') {x=x*10+ch-'0'; ch=getchar();} 14 return x*f; 15 } 16 double dp[1010][1010]; 17 int main() 18 { 19 int n,s; 20 double p1,p2,p3,p4; 21 while (scanf("%d%d",&n,&s)!=EOF) 22 { 23 for (int i=0; i<=n; i++) for (int j=0; j<=s; j++) dp[i][j]=0; 24 for (int i=n; i>=0; i--) 25 for (int j=s; j>=0; j--) 26 { 27 if (i==n && j==s) continue; 28 p1=(double)i/n*j/s*1.0; 29 p2=(double)(n-i)/n*j/s*1.0; 30 p3=(double)i/n*(s-j)/s*1.0; 31 p4=(double)(n-i)/n*(s-j)/s*1.0; 32 dp[i][j]=(p2*dp[i+1][j]+p3*dp[i][j+1]+p4*dp[i+1][j+1]+1.0)/(1.0-p1); 33 } 34 printf("%.4f ",dp[0][0]); 35 } 36 return 0; 37 }