1.3.1Mixing Milk

  
    

   
     
/*
   
     
ID: awsd1231
   
     
PROG: milk
   
     
LANG: C++
   
     
*/
   
     
#include<iostream>
   
     
#include<cstdio>
   
     
#include<map>
   
     
#include<algorithm>
   
     

   
     
using namespace std;
   
     

   
     
const int maxn =5000 + 10;
   
     
int n, m;
   
     
struct Pa {
   
     
 int p;
   
     
 int pa;
   
     
};
   
     
Pa pa[maxn];
   
     

   
     
bool operator < ( Pa a, Pa b) {
   
     
 if(a.p < b.p) return true;
   
     
 else return false;
   
     
}
   
     

   
     
int main() {
   
     
 freopen("milk.in", "r", stdin);
   
     
 freopen("milk.out", "w", stdout);
   
     
 cin >> n >> m;
   
     
 for(int i = 0; i != m; ++i) 
   
     
 cin >> pa[i].p >> pa[i].pa;
   
     
 sort(pa, pa+m);
   
     
 int need = n, cost = 0, idx = 0;
   
     
 while(need > 0) {
   
     
 cost += pa[idx].p * pa[idx].pa;
   
     
 need -= pa[idx++].pa;
   
     
 }
   
     
 cost -= pa[--idx].p * pa[idx].pa;
   
     
 need += pa[idx].pa;
   
     
 while(need--)
   
     
 cost += pa[idx].p;
   
     
 cout << cost << endl;
   
     
 return 0;
   
     
} 
  
    

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