#include <bits/stdc++.h>
using namespace std;
int f[10010],l,m,ans;
int main() {
    memset (f,0,sizeof (f));
    cin>>l>>m;
    for (int i=1;i<=m;i++) {
        int x,y;
        cin>>x>>y;
        for (int j=x;j<=y;j++) f[j]=1;
    }
    for (int i=0;i<=l;i++) 
      if (!f[i]) ans++;
    cout<<ans;
    return 0;
}