#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
int n,m,rank[11000],a[11000],p[11000],ans;
bool cmp(int x,int y) {
	return p[x]<p[y];
}
int main() {
	freopen("milk.in","r",stdin);
	freopen("milk.out","w",stdout);
	scanf("%d%d",&n,&m);
	for(int i=1; i<=m; i++) {
		scanf("%d%d",&p[i],&a[i]);
		rank[i]=i;
	}
	sort(rank+1,rank+1+m,cmp);
	for(int i=1; i<=m&&n>0; i++) {
		int j=rank[i];
		if(n>=a[j]) {
			n-=a[j];
			ans+=a[j]*p[j];
		} else {
			ans+=n*p[j];
			n=0;
		}
	}
	printf("%d\n",ans);
	fclose(stdin);
	fclose(stdout);
}