#include<fstream>
using namespace std;
ofstream fout("milk.out");
ifstream fin("milk.in");
int a,c,d,e,f,g,h,i;
int b[5000][2];
int main()
{
	fin>>a>>c;
	for(i=0;i<c;i++){
		fin>>b[i][0]>>b[i][1];
	}
	g=a;
	for(d=0;d<c;d++){
		for(e=0;e<c-d;e++){
			if(b[e][0]>b[e+1][0]){
				f=b[e][0];b[e][0]=b[e+1][0];b[e+1][0]=f;
				f=b[e][1];b[e][1]=b[e+1][1];b[e+1][1]=f;}}}
	for(e=0;e<a;e++){
		if(b[e][1]<g){
			h+=b[e][0]*b[e][1];
			g-=b[e][1];}
		else{
			h+=b[e][0]*g;
			e=a;}}
	fout<<h;
	fin.close();
	fout.close();
	return 0;
}