#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=1e6+10;
const ll Inf=1e17;
int ID,n,m;
ll L,s[N],t[N];
struct que{
	ll k;int x;
	bool operator<(const que h)const{return k<h.k;}
}q[N];
ll ans[N];
struct line{
	ll l,r;
	bool operator<(const line h)const{return l<h.l;}
}a[N],b[N],c[N];
ll f[N],g[N],rr[N];
void tomin(ll &x,ll y){if(x>y)x=y;}
set<pair<ll,int> > ss;
void work(){
	ll mn=L,mx=0;
	int n1=0,n2=1;
	for(int i=1;i<=n;i++){
		mn=min(mn,min(s[i],t[i]));
		mx=max(mx,max(s[i],t[i]));
		if(t[i]<s[i])a[++n1]={t[i],s[i]},rr[n1]=s[i];
		c[i]={s[i],t[i]};
	}
	sort(rr+1,rr+n1+1);
	sort(c+1,c+n+1);
	sort(a+1,a+n1+1);
	b[1].l=b[1].r=mn;
	for(int i=1;i<=n1;i++){
		if(b[n2].r<=a[i].l)b[++n2]=a[i];
		else b[n2].r=max(b[n2].r,a[i].r);
	}	
	b[n2+1].l=b[n2+1].r=mx,f[n2+1]=0;
	++n2;
	b[n2+1].l=b[n2+1].r=mx,f[n2+1]=0;
	for(int i=n2;i>=1;i--){
		f[i]=min(2*b[i].r-3*b[i].l+b[i+1].l+f[i+1],2*(mx-b[i].l));
	}
	g[n1+1]=Inf;
	ll mc=mx;
	int t=n,p=n2;
	for(int i=n1;i>=1;i--){
		while(t>=1&&c[t].l>rr[i])mc=min(mc,c[t].r),--t;
		mc=min(mc,rr[i]);
		while(p>1&&b[p-1].r>rr[i])--p;
		if(mc>=b[p].l)g[i]=min(g[i+1],2*rr[i]-2*mn+mc+min(2*b[p].r-3*mc+b[p+1].l+f[p+1],2*(mx-mc)));
		else g[i]=min(g[i+1],2*rr[i]-2*mn+b[p].l+f[p]);
//		if(mc>=b[p].l)printf("%lld %lld %lld %lld %lld\n",rr[i],mc,b[p].l,b[p].r,rr[i]-2*mn+mc+min(2*b[p].r-3*mc+b[p+1].l+f[p+1],2*(mx-mc)));
//		else printf("%lld %lld %lld %lld %lld\n",rr[i],mc,b[p].l,b[p].r,rr[i]-2*mn+b[p].l+f[p]);
	}
//	printf("%d %d\n",n1,n2);
//	puts("");
	ss.clear();
	for(int i=1;i<=n;i++)if(c[i].l>c[i].r)ss.insert(make_pair(c[i].r,i));
	ss.insert(make_pair(mx,0));
	t=1,p=1;
	int tt=1;
	sort(q+1,q+m+1);
	for(int i=1;i<=m;i++){
		if(q[i].k>=mx)break;
		if(q[i].k<=mn){
			tomin(ans[q[i].x],mn-q[i].k+f[1]);
			continue;
		}
		while(b[p].r<q[i].k)++p;
		while(t<=n&&c[t].l<=q[i].k){
			if(c[t].l>c[t].r)ss.erase(make_pair(c[t].r,t));
			++t;
		}
		while(tt<=n1&&rr[tt]<q[i].k)++tt;
		tomin(ans[q[i].x],-q[i].k+g[tt]);
		ll lp=ss.begin()->first;
		if(lp<=b[p].r)tomin(ans[q[i].x],lp+q[i].k-2*mn+min(2*b[p].r-3*lp+b[p+1].l+f[p+1],2*(mx-lp)));
//		printf("%lld %lld\n",g[tt]-q[i].k,lp+q[i].k-2*mn+min(2*b[p].r-3*lp+b[p+1].l+f[p+1],2*(mx-lp)));
	}
}
signed main(){
//	freopen("bus.in","r",stdin);
//	freopen("bus.out","w",stdout);
	scanf("%lld%d",&L,&n);m=n;
	for(int i=1;i<=n;i++)scanf("%lld%lld",&s[i],&t[i]);
	for(int i=1;i<=m;i++)q[i].k=s[i],q[i].x=i,ans[i]=Inf;
	work();
	for(int i=1;i<=n;i++)s[i]=L-s[i]+1,t[i]=L-t[i]+1;
	for(int i=1;i<=m;i++)q[i].k=L-q[i].k+1;
	work();
	ll Ans=ans[1];
	for(int i=1;i<=m;i++)Ans=min(Ans,ans[i]);
	printf("%lld\n",Ans);
}
/*
1 5 1 10
6 4
5 10
6 1
5 8
1 5
5
*/