分析：水题，按照价格从小到大排序，在进行贪心即可

/*    PROB:milk    ID:wanghan    LANG:C++*/#include "iostream"#include "cstdio"#include "cstring"#include "string"#include "algorithm"using namespace std;const int maxn= 5000+10;struct Node{    int p,a;};Node h[maxn];bool cmp(Node x,Node y){    if(x.p==y.p)        return x.a>y.a;    return x.p
      
       >n>>m;    for(int i=0;i
       
        >h[i].p>>h[i].a;    sort(h,h+m,cmp);    long long ans=0;    int flag=0;    /*if(n>=h[0].a){        n-=h[0].a;        ans+=(h[0].p*h[0].a);        if(n==0) flag=1;    }else{        flag=1;        ans+=(h[0].a-n)*h[0].p;        //n=-1;    }*/    //cout<
        
         <
         
          =h[i].a){            n-=h[i].a;            ans+=(h[i].a*h[i].p);            if(n==0)  flag=1;        }else{            ans+=n*h[i].p;            n=-1;            flag=1;        }    }    cout<
          
           <