package Main;
import java.util.*;
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner (System.in);
int brokenString = in.nextInt();
int brand = in.nextInt();
int minPackage = in.nextInt();
int minIdividual = in.nextInt();
for (int i = 0; i < brand - 1 ;i++) {
int inputPackage = in.nextInt();
int inputIdividual = in.nextInt();
if (inputPackage < minPackage)
minPackage = inputPackage;
if (inputIdividual < minIdividual)
minIdividual = inputIdividual;
}
if (minPackage/6 >= minIdividual)
System.out.println(brokenString * minIdividual);
else {
int result = brokenString / 6 * minPackage;
brokenString %= 6;
if (brokenString * minIdividual > minPackage) {
result += minPackage;
}
else {
result += brokenString * minIdividual;
}
System.out.println(result);
}
}
}
https://www.acmicpc.net/problem/1049