cf 398B. Painting The Wall

23333，还是不会。。%%%http://hzwer.com/6276.html

 #include <bits/stdc++.h>
 #define LL long long
 #define lowbit(x) x&(-x)
 #define inf 0x3f3f3f3f
 using namespace std;
 inline int ra()
 {
     int x=,f=; char ch=getchar();
     while (ch<'' || ch>'') {if (ch=='-') f=-; ch=getchar();}
     while (ch>='' && ch<='') {x=x*+ch-''; ch=getchar();}
     return x*f;
 }
 double dp[][];
 int n,m,t1,t2;
 bool r[],c[];
 int main(int argc, char const *argv[])
 {
     n=ra(); m=ra();
     t1=t2=n;
     for (int i=; i<=m; i++)
     {
         int x=ra(),y=ra();
         if (!r[x]) r[x]=,t1--;
         if (!c[y]) c[y]=,t2--;
     }
     for (int i=; i<=n; i++)
     {
         dp[i][]=dp[i-][]+(double)n/i;
         dp[][i]=dp[][i-]+(double)n/i;
     }
     for (int i=; i<=t1; i++)
         for (int j=; j<=t2; j++)
         {
             dp[i][j]=n*n;
             dp[i][j]+=i*j*dp[i-][j-];
             dp[i][j]+=i*(n-j)*dp[i-][j];
             dp[i][j]+=j*(n-i)*dp[i][j-];
             dp[i][j]/=(n*n-(n-i)*(n-j));
         }
     printf("%.10lf\n",dp[t1][t2]);
     return ;
 }