#include <bits/stdc++.h>
using namespace std;
#define ll long long

int main()
{
    int n, m; cin >> n >> m;
    vector <pair<int, int>> P(m);
    for(int i = 0; i < m; i++){
       cin >> P[i].first >> P[i].second;
       P[i].first--;
       P[i].second--;
    }
    vector <vector <bool>> A(m+1,vector <bool>(n, false));
    for(int i = 0; i < m; i++){
        for(int j = 0; j < n; j++)
        {
            if(A[i][j]) A[i+1][j] = true;
            else{
                if(j <= P[i].second && j >= P[i].first) A[i+1][j] = true;
            }
        }
    }
    int ans = 0;
    for(int i = 0; i < n; i++)
    {
        if(!A[m][i]) ans++;
    }
    cout << ans;
    return 0;
}