#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 10000;
int L, n;
int main()
{
bool a[N];
int count = 0;
cin >> L >> n;
for(int i = 0; i <= L; i++) a[i] = true;
while(n--)
{
int c, d;
cin >> c >> d;
for(int i = c; i <= d; i++)
{
a[i] = false;
}
}
for(int i = 0; i <= L; i++)
{
count += a[i];
}
cout << count;
}