/*
ID: mytbk921
LANG: C
TASK: milk
*/

#include <stdio.h>
#include <stdlib.h>
#define MAXF 5000

typedef struct
{
	int price;
	int amount;
}FARMER;

int cmp(const void* x,const void* y)
{
	return ((FARMER*)x)->price-((FARMER*)y)->price;
}

int main()
{
	int Total,nFarmers,minPrice;
	FARMER farmers[MAXF];
	int i;
	FILE *fin=fopen("milk.in","r");
	FILE *fout=fopen("milk.out","w");

	fscanf(fin,"%d%d",&Total,&nFarmers);
	for (i=0;i<nFarmers;i++)
		fscanf(fin,"%d%d",&farmers[i].price,&farmers[i].amount);
	fclose(fin);
	qsort(farmers,nFarmers,sizeof(FARMER),cmp);

	minPrice=0;
	for (i=0;Total&&i<nFarmers;i++){
		if (farmers[i].amount>Total){
			minPrice+=Total*farmers[i].price;
			Total=0;
		}
		else{
			minPrice+=farmers[i].price*farmers[i].amount;
			Total-=farmers[i].amount;
		}
	}

	fprintf(fout,"%d\n",minPrice);
	fclose(fout);
	return 0;
}