#include<bits/stdc++.h>
using namespace std;
int arr[10000] = {0};
int main(){
    int l, m;
    cin >> l>>m;
    int x,y;
    int ans = 0;
    for (int i = 0;i<m;i++){
        cin >> x>> y;
            for(int j = x;x<=y;j++){
                arr[x] = 1;
        }

}

    for (int k = 0; k<=l;k++){
        if(arr[k]== 0)
            ans++;


    }
    cout << ans;
    return 0;
}