include <bits/stdc++.h>

using namespace std;

int main() {

int a,b;
cin>>a>b;
int l[a];
int m[b+1],n[b+1];
int ans=a;
for(int i=0;i<b+2;i++)
    l[i]==0;
for(int i=0,i<b,i++)
{
    cin>>m[i]>>n[i];
    if(i<=1)
    {
        for(int z=m[i];z<=n[i];z++)
            l[z]==0;
        continue;
    }
    else
    {
        if(m[i]>=m[i-1];n[i]<=n[i-1])
            continue;
        else if(m[i]>=m[i-1];n[i]>n[i-1])
        {
            for(int z=m[i];z>m[i-1];z--)
                l[z]==0;
            continue;
        }
        else if(m[i]<[i-1];n[i]<=n[i-1])
        {
            for(int z=n[i-1];z>=n[i];z--)
                l[z]==0;
            continue;
        }
        else if(m[i]<[i-1];n[i]>n[i-1])
        {
            for(int z=m[i];z>m[i-1];z--)
                l[z]==0;
            for(int o=n[i-1];o>=n[i];o--)
                l[o]==0;
            continue;
        }
    }
}
for(int i=0;i<=a)
{
    if(l[i]==0)
    ans--;
}
cout<<ans;
return 0;

}