/*
ID: cjn77881
LANG: C++
TASK: milk
*/

#include <stdio.h>

typedef struct Rec Rec;
struct Rec{
	int p,a;
};

int sumA,numF,tmp,idx,ans;
Rec rec[5000];

void swap(Rec &a,Rec &b){
	Rec c=a;
	a=b;
	b=c;
	return;
}

int min(int x,int y){
	return x>y?y:x;
}

void qSort(Rec *a,int h,int t){
	if
 
 (h>=t) return;
	int x = a[(h+t)>>1].p;
	int i=h,j=t;
	while (i<=j){
		while (a[i].p<x) i++;
		while (a[j].p>x) j--;
		if (i<=j){
			swap(a[i],a[j]);
			i++; 
			j--;
		}
	}
	qSort(a,h,j);
	qSort(a,i,t);
	return;
}

int main(){
	
	FILE *fin = fopen("milk.in","r");
	FILE *fout = fopen
 
("milk.out","w");
	
	fscanf(fin,"%d %d\n",&sumA,&numF);
	for (int i=0;i<numF;i++) fscanf(fin,"%d %d\n",&rec[i].p,&rec[i].a);
	qSort(rec,0,numF-1);
	//for (int i=0;i<numF;i++) printf("%d %d\n",rec[i].p,rec[i].a);
	while (sumA>0){
		tmp=min(sumA,rec[idx].a);
		ans+=rec[idx].p*tmp;
		sumA-
 
=tmp;
		idx++;
	}
	fprintf(fout,"%d\n",ans);
	return 0;
}