#include<bits/stdc++.h>

#define ll long long
#define mk make_pair
#define fi first
#define se second
#define int long long

using namespace std;

inline ll read(){
	ll x=0,f=1;char c=getchar();
	for(;(c<'0'||c>'9');c=getchar()){if(c=='-')f=-1;}
	for(;(c>='0'&&c<='9');c=getchar())x=x*10+(c&15);
	return x*f;
}

const int mod=998244353;
int ksm(int x,int y,int p=mod){
	int ans=1;
	for(int i=y;i;i>>=1,x=1ll*x*x%p)if(i&1)ans=1ll*ans*x%p;
	return ans%p;
}
int inv(int x,int p=mod){return ksm(x,p-2,p)%p;}
mt19937 rnd(time(0));
int randint(int l,int r){return rnd()%(r-l+1)+l;}
void add(int &x,int v){x+=v;if(x>=mod)x-=mod;}
void Mod(int &x){if(x>=mod)x-=mod;}
int cmod(int x){if(x>=mod)x-=mod;return x;}

void cmax(ll &x,ll v){x=max(x,v);}
void cmin(ll &x,ll v){x=min(x,v);}

const int N=1e6+5;
int n,pre[N],pos[N],op[N],m,len;
int L[N],R[N],a[N],b[N],c[N],d[N];
vector<pair<int,int> >A,B;

signed main(void){

#ifndef ONLINE_JUDGE
	freopen("game.in","r",stdin);
#endif
	// freopen("sample_game2.in","r",stdin);
	// freopen("game.out","w",stdout);

	len=read(),n=read();
	// for(int i=1;i<=n;i++){
		// pos[i]=read();
		// op[i]=read();
		// if(op[i]==2)pre[i]=read();
	// }
	// for(int i=1;i<=n;i++)if(op[i]==1)L[i]=R[i]=pos[i];
	// for(int i=1;i<=n;i++)if(op[i]==2)cmin(L[pre[i]],pos[i]),cmax(R[pre[i]],pos[i]);

	// for(int i=1;i<=n;i++)if(op[i]==1){
	// 	// cout<<"i = "<<i<<" L,R = "<<L[i]<<" "<<R[i]<<endl;
	// 	A.emplace_back(mk(pos[i],R[i]));
	// 	if(L[i]<pos[i])B.emplace_back(mk(L[i],pos[i]));
	// }

    for(int i=1;i<=n;i++){
        int l=read(),r=read();
        if(l<r)A.emplace_back(mk(l,r));
        else B.emplace_back(mk(r,l));
    }

	sort(A.begin(),A.end()),sort(B.begin(),B.end());
	auto Rec=[&](vector<pair<int,int> >&C){
		int tl=-1,tr=-1;
		vector<pair<int,int> >R;
		for(auto [l,r]:C){
			// cout<<"l,r = "<<l<<" "<<r<<" tl,tr = "<<tl<<" "<<tr<<endl;
			if(l>tr){
				if(tl!=-1)R.emplace_back(mk(tl,tr));
				tl=l,tr=r;
			}
			else cmax(tr,r);
		}
		if(tl!=-1)R.emplace_back(mk(tl,tr));
		C=R;
	};
	Rec(A),Rec(B);

	// cout<<"A = ";for(auto [l,r]:A)cout<<"["<<l<<","<<r<<"] ";puts("");
	// cout<<"B = ";for(auto [l,r]:B)cout<<"["<<l<<","<<r<<"] ";puts("");

	if(A.size()==0){
		cout<<B.back().se-B[0].fi<<endl;
		return 0;
	}
	if(B.size()==0){
		cout<<A.back().se-A[0].fi<<endl;
		return 0;
	}

	n=A.size(),m=B.size();
	for(int i=1;i<=n;i++)a[i]=A[i-1].fi,b[i]=A[i-1].se;
	for(int i=1;i<=m;i++)c[i]=B[i-1].fi,d[i]=B[i-1].se;

	// puts("ok");

	ll ans=1e18;
	vector<ll>sum(max(n,m)+5),vx(max(n,m)+5),vy(max(n,m)+5);

	for(int i=1;i<=m;i++)sum[i]=sum[i-1]+d[i]-c[i];
	for(int i=1;i<=m;i++)sum[i]*=2;
	for(int i=1;i<=m;i++)vx[i]=d[i]-c[1]+abs(c[1]-a[1]),vy[i]=d[m]-c[i]+abs(d[m]-b[n]);
	if(c[1]<a[1])vx[0]=1e14;
	if(d[m]>b[n])vy[m+1]=1e14;
	int mn=vx[0]-sum[0];
	for(int i=1;i<=m+1;i++){
		ans=min(ans,b[n]-a[1]+vy[i]+sum[i-1]+mn);
		// cout<<"i = "<<i<<" vx = "<<vx[i]<<" vy = "<<vy[i]<<" -> ans = "<<ans<<endl;
		// cout<<"mn = "<<mn<<" sum = "<<sum[i-1]<<endl;
		mn=min(mn,vx[i]-sum[i]);
	}
	// cout<<"now ans = "<<ans<<endl;
	// for(int i=0;i<=m;i++)for(int j=i+1;j<=m+1;j++)ans=min(ans,b[n]-a[1]+vx[i]+vy[j]+sum[j-1]-sum[i]);

	fill(sum.begin(),sum.end(),0),fill(vx.begin(),vx.end(),0),fill(vy.begin(),vy.end(),0);
	for(int i=1;i<=n;i++)sum[i]=sum[i-1]+b[i]-a[i];
	for(int i=1;i<=n;i++)sum[i]*=2;
	for(int i=1;i<=n;i++)vx[i]=b[i]-a[1]+abs(a[1]-c[1]),vy[i]=b[n]-a[i]+abs(b[n]-d[m]);
	if(a[1]<c[1])vx[0]=1e14;
	if(b[n]>d[m])vy[n+1]=1e14;
	mn=vx[0]-sum[0];
	for(int i=1;i<=n+1;i++){
		ans=min(ans,d[m]-c[1]+vy[i]+sum[i-1]+mn);
		mn=min(mn,vx[i]-sum[i]);
	}
	// for(int i=0;i<=n;i++)for(int j=i+1;j<=n+1;j++)ans=min(ans,d[m]-c[1]+vx[i]+vy[j]+sum[j-1]-sum[i]);

	cout<<ans<<endl;

	return 0;
}