#include<bits/stdc++.h>
using namespace std;
int n,m,now,cost;
struct milker{
	int prize,have;
}a[2000005];
bool cmp(milker a,milker b){
	return a.prize<b.prize;
}
int main(){
	freopen("milk.in","r",stdin);
	freopen("milk.out","w",stdout);
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++){
		scanf("%d%d",&a[i].prize,&a[i].have);
	}
	sort(a+1,a+n+1,cmp);
	int x=1;//表示现在已经买到第几个奶农了 
	while(n>now){
		if(a[x].have>(n-now)){
			cost+=(n-now)*a[x].prize;
			break;
		}
		now+=a[x].have;
		cost+=a[x].prize*a[x].have;
		x++;
	}
	printf("%d\n",cost);
	return 0;
}

在USACO上就是这样的： Run 1: Execution error: Your program exited with exit status `127'.