#include <stdio.h>
#include <string.h>
int main()
{
int l,m,n,begin,end;
scanf("%d",&l);
scanf("%d",&m);
n=l+1;
int a[n];
for (int i=0;i<n;i++)
a[i]=0;
while (m>0)
{
scanf("%d",&begin);
scanf("%d",&end);
for (int i=begin-1;i<end;i++)
{
a[i]=a[i]+1;
}
m-=1;
}
int count=0;
for (int i=0;i<n;i++)
{
if (a[i]==0) count++;
}
printf("%d",count);
return 0;
}
