import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner=new Scanner(System.in);
		int n=scanner.nextInt();
		int m=scanner.nextInt();
		int max=0;
		for (int i = 0; i < m; i++) {
			int a=scanner.nextInt();
			int b=scanner.nextInt();
			if(a>n) {
				continue;
			}else {
				if(max<b)max=b;
			}
		}
		System.out.println(n-max);
		

	}

}

这题没啥好说的,判断能不能使用这张券,能使用就记录最大的优惠,最后输出n-max