import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Ani_Bus {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        int patnici = Integer.parseInt(reader.readLine());
        int spirki = Integer.parseInt(reader.readLine());


        for (int i = 1; i <= spirki; i++) {
            int slizat = Integer.parseInt(reader.readLine());
            int ka4vatSa = Integer.parseInt(reader.readLine());

            if (i % 2 != 0){
                patnici += 2;
            } else if (i % 2 == 0){
                patnici -= 2;
            }
            patnici += ka4vatSa;
            patnici -= slizat;

        }

        System.out.println(patnici);


    }
}