2 2
3 1
2 3
0

5

AC-code：
#include<stdio.h>
#include<algorithm>
using namespace std;

struct bao
{
	int pi,mi;
}ba[105];

bool cmp(bao a,bao b)
{
	return a.pi>b.pi;
}

int main()
{
	int v,n,i,sum;
	while(scanf("%d%d",&v,&n),v)
	{
		for(i=0;i<n;i++)
			scanf("%d%d",&ba[i].pi,&ba[i].mi);
		sort(ba,ba+n,cmp);
		sum=0;
		for(i=0;i<n;i++)
		{
			if(ba[i].mi<=v)
			{
				sum+=ba[i].pi*ba[i].mi;
				v-=ba[i].mi;
			}
			else
			{
				sum+=v*ba[i].pi;
				break;
			}
		}
		printf("%d\n",sum);
	}
	return 0;
}