C :

#include<stdio.h>
int m,n,a[1100][2],ans;
int main()
{
	scanf("%d %d",&m,&n);
	for(int i=1;i<=n;i++)
	scanf("%d %d",&a[i][0],&a[i][1]);
	for(int i=1;i<n;i++)
	for(int j=i+1;j<=n;j++)
	if(a[i][0]>a[j][0])
	{
		int t1=a[i][0],t2=a[i][1];
		a[i][0]=a[j][0];
		a[j][0]=t1;
		a[i][1]=a[j][1];
		a[j][1]=t2;
	}
	for(int i=1;i<=n&&m>0;i++)
		if(a[i][1]>=m)
		{
			ans+=m*a[i][0];
			m=0;
		}
		else
		{
			ans+=a[i][1]*a[i][0];
			m-=a[i][1];
		}
		printf("%d",ans);
		return 0;
}

Java :

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int m, n;
        m = sc.nextInt();
        n = sc.nextInt();
        List<Map<String, Integer>> shopList = new ArrayList<>();
        for (int i = 1; i <= n; i++) {
            Map<String, Integer> shop = new HashMap<>();
            shop.put("price", sc.nextInt());
            shop.put("count", sc.nextInt());
            shopList.add(shop);
        }
        //容器排序
        Collections.sort(shopList, new Comparator<Map<String, Integer>>() {
            @Override
            public int compare(Map<String, Integer> o1, Map<String, Integer> o2) {
                //如果前一个 price > 后一个 price，则交换这两个map
                return o1.get("price") - o2.get("price");
            }
        });
        //总共需要购买 m 个贺卡，m 还需要购买的贺卡个数
        int money = 0;
        for (int i = 0; i < shopList.size(); i++) {
            //第 i 间商铺
            Map<String, Integer> shop = shopList.get(i);
            if( shop.get("count") >= m){
                //不需要这么多了
                money = money + m * shop.get("price");
                m = 0;
                break;
            }else{
                money = money + shop.get("count") * shop.get("price");
                m = m - shop.get("count");
            }
        }
        System.out.print(money);
        sc.close();
    }
}

Python :

'''
stus = []
class Student:
    def __init__(self,id,data,ave):
        self.id = id
        self.data= data
        self.ave=ave
n=int(input())
for i in range(n):
  t=list(map(int,input().split()))
  x=t[0:1]
  y=t[1:]
  y=sorted(y)
  stus.append(Student(id=x[0],data=y,ave=sum(y[1:4])/3))
stulist = sorted(stus, key=lambda zz:(-zz.ave, zz.id))
for i in range(3):
  print("%d %.3lf"%(stulist[i].id,stulist[i].ave))



    
'''  


class Hk:
  def __init__(self,price,num):
    self.price=price
    self.num=num
hks=[]

total,n=list(map(int,input().split()))
for i in range(n):
  price,num=list(map(int,input().split()))
  hks.append(Hk(price,num))

hklist = sorted(hks,key=lambda t:(t.price))
hksum=0
pricesum=0
i=0

  
while hksum+hklist[i].num<total:
  hksum=hksum+hklist[i].num
  pricesum=pricesum+hklist[i].price*hklist[i].num
  i=i+1
else:
  pricesum=pricesum+hklist[i].price*(total-hksum)
print(pricesum)