[51nod] 1450 闯关游戏

http://www.51nod.com/onlineJudge/questionCode.html#problemId=1450&noticeId=462195

f[i][j]表示玩了前i个游戏，还剩j次必须达到两金币

不是很理解为什么自己推的式子里分子是2

#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>

using namespace std;

inline int rd(){
  int ret=0,f=1;char c;
  while(c=getchar(),!isdigit(c))f=c=='-'?-1:1;
  while(isdigit(c))ret=ret*10+c-'0',c=getchar();
  return ret*f;
}

const int MAXN=2048;

double f[MAXN*2][MAXN];
int n,m;
struct Node{
  int x,y;
  bool operator < (const Node &rhs)const{
    return y==rhs.y?x>rhs.x:y>rhs.y;
  }
}a[MAXN];
inline void upmin(double &x,double y){
  x=min(x,y);
}
int main(){
  n=rd();m=rd();
  for(int i=1;i<=n;i++){
    a[i].x=rd();a[i].y=rd();
  }
  memset(f,0x7f,sizeof(f));
  sort(a+1,a+1+n);
  m-=n;f[0][m]=0;
  for(int i=1;i<=n;i++){
    double x=a[i].x*1.0,y=a[i].y*1.0,s=x+y;
    for(int j=0;j<m;j++){
      upmin(f[i][j],f[i-1][j]*x/s+f[i-1][j+1]*y/s+1000.0/s);
      upmin(f[i][j],f[i-1][j+1]+1000.0/y);
    }
    upmin(f[i][m],f[i-1][m]+1000.0/s);
  }
  printf("%.7lf",f[n][0]);
  return 0;
}