#include<iostream>
using namespace std;
int main(){
    int l = 0,m = 0,u = 0,v = 0;
    cin >> l >> m;
    int tree = l + 1;
    int arr[tree];
    for(int i = 0;i < m;i++){
        cin >> u >> v;
        for(int i = u;i <= v;i++){
            arr[i] = 1;
        }
    }
    int count = 0;
    for(int i = 0;i < tree;i++){
        if(arr[i] == 1){
            count++;
        }
    }
    tree -= count;
    cout << tree;
    return 0;
}