#include <iostream>
#include<string>
#include<sstream>
#include<algorithm>
#include <math.h>
#include <set>
#include <map>
#include <vector>
#include <set>
//#include <unordered_set>
#define ll long long
using namespace std;
ll a[10005] = {0};

int main()
{
    ll l,m;
    cin>>l>>m;
    for(ll i=1;i<=m;i++){
        ll x,y;
        cin>>x>>y;
        for(ll j = x;j<=y;j++){
            a[j]=1;
        }
    }

    ll sum = 0;
     for(ll j = 0;j<=l;j++){
            if(a[j]==0){
                sum++;
            }
        }
        cout<<sum;

    return 0;
}