#include<iostream>
using namespace std;
int main(){
	int n,l;
	int a,b;
	int num = 0;
	cin >> n >> l;
	int arr[n];
	for(int i = 0;i <= n;i++){
		arr[i] = 1;
	}
	for(int i = 1;i <= l;i++){
		cin >> a >> b;
		for(int j = a;j <= b;j++){
			if(arr[j] == 1){
				num++;
				arr[j]++;
			}
		}
	}
	cout << n-num << endl;
	return 0;
}