#include <bits/stdc++.h>
using namespace std;
struct milk{
	int a,b;
}a[1000001];
bool cmp(milk x,milk y){
	return x.a<=y.a;
}
int main(){//因为N范(shu)围(ju)很(hen)小(shui) 
	int n,m;
	cin>>m>>n;
	
	for(int i=1;i<=n;i++){
		cin>>a[i].a>>a[i].b;
	}
	sort(a+1,a+n+1,cmp);
	int sum=0;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=a[i].b;j++){
			m--;
			//cout<<m<<endl;
			sum+=a[i].a;
			if(m<=0)break;
		}
		if(m<=0)break;
	}
	cout<<sum<<endl;
}  

RE #6 爆循环 本人小学生，请dalao多多指教