import java.util.Arrays;
import java.util.HashMap;
import java.util.Scanner;
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String inStr = sc.nextLine();
int cost = Integer.parseInt(inStr.split(" ")[0]);// 花的钱
int n = Integer.parseInt(inStr.split(" ")[1]);// n张优惠券
int[] arr = new int[n];
for (int i = 0; i < n; i++) {
String inStr1 = sc.nextLine();
int money = Integer.parseInt(inStr1.split(" ")[0]);
int money1 = Integer.parseInt(inStr1.split(" ")[1]);
if(money <= cost) {
arr[i] = money1;
}
}
Arrays.sort(arr);
System.out.println(cost-arr[arr.length-1]);
}
}