#include <cstdio>
#include <cstring>
#include <fstream>
#include <iostream>
#include <algorithm>
using namespace std;
ifstream fin("milk.in");
ofstream fout("milk.out");
int N,M,T,ANS;
struct PERSON{
	int P,A;
}p[5010];
inline bool COMP(PERSON a,PERSON b) { return a.P<b.P; }
int main(){
	fin>>N>>M;
	for(int i=1;i<=M;++i)
	    fin>>p[i].P>>p[i].A;
	sort(p+1,p+M+1,COMP);
	for(int i=1;i<=M;++i)
		if(T+p[i].A>=N){
			ANS+=(N-T)*p[i].P;
			break;
		}
		else{
			T+=p[i].A;
			ANS+=(p[i].A*p[i].P);
		}
	fout<<ANS<<endl;
	return 0;
}