import java.util.Scanner;

public class bus {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int countPassengersBeginning = Integer.parseInt(scanner.nextLine());
        int countStops = Integer.parseInt(scanner.nextLine());
        int newPassengers = 0;
        int totalPassengers = 0;

        for (int i = 1; i <= countStops; i++) {

            int slizashti = Integer.parseInt(scanner.nextLine());
            newPassengers = countPassengersBeginning - slizashti;
            if (i % 2 == 1){
                newPassengers +=2;
            } else if (i % 2 == 0) {
                newPassengers -= 2;
            }

            int kachvashti = Integer.parseInt(scanner.nextLine());
            newPassengers = countPassengersBeginning + kachvashti;
            if (i % 2 == 1){
                newPassengers +=2;
            } else if (i % 2 == 0) {
                newPassengers -= 2;
            }


        }
        System.out.println(newPassengers);

    }
}