学步园

#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
struct data{
	int c,v;
}e[5001];
bool cmp(data a,data b){
	if(a.c<b.c)return 1;
	else return 0;
}
int n,m,ans;
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++)
		scanf("%d%d",&e[i].c,&e[i].v);
	sort(e+1,e+m+1,cmp);
	for(int i=1;i<=m;i++){
		ans+=min(n,e[i].v)*e[i].c;
		n=n-min(n,e[i].v);
		if(!n)break;
	}
	printf("%d",ans);
	return 0;
}