62 1 7 8wa

#include <bits/stdc++.h>
using namespace std;
int n;
int t,num;
struct albb{
	int w,m;
}a[10086];
bool cmp(albb xx,albb yy)
{
	return xx.m<yy.m;
}
int main()
{
	cin>>t>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>a[i].m>>a[i].w;
	}
	sort(a+1,a+n+1,cmp);
	for(int i=1;i<=n;i++)
	{
		if(t==0)   break;                         
		if(a[i].w<=t)
		{
			t-=a[i].w;
			num+=a[i].m*a[i].w;
		}
		else if(a[i].w>t)
		{
			t=0;
			num+=a[i].m*t;
		}
	}
	cout<<num;
	return 0;
 }