C++ :

#include<cstdio>
#include<algorithm>
using namespace std;
struct p
{
	int num;
	int pr;
}a[1005];
int comp(const p &a,const p &b)
{
	if(a.pr<b.pr)return 1;
	else return 0;
}
int main()
{
	int m,n;
	scanf("%d%d",&m,&n);
	for(int i=1;i<=n;++i)
	 scanf("%d%d",&a[i].pr,&a[i].num);
	sort(a+1,a+n+1,comp);
	int i=0,ans=0;
	while(m>0)
	{
		++i;
		if(m>=a[i].num)
		{
			ans+=a[i].num*a[i].pr;
			m-=a[i].num;
			continue;
		}
		if(m<=a[i].num)
		{
			ans+=m*a[i].pr;
			m=0;
			continue;
		}
	}
	printf("%d",ans);
	return 0;
}