#include <bits/stdc++.h>
using i64 = long long;

const int maxn = 1e5 + 5;
const i64 INF = 1e16;
struct node {
	int x1,y1,x2,y2,id;
	node() {
		x1 = y1 = x2 = y2 = id = 0;
	}
	node(int x1,int y1,int x2,int y2):x1(x1),y1(y1),x2(x2),y2(y2){};
} a[maxn];
int n,cnt;
i64 m,c[maxn << 2],f[maxn];

struct BIT {
	i64 c[maxn << 2];
	BIT() {
		for(int i = 0;i < (maxn << 2);++ i)
			c[i] = INF;
	}
	int lowbit(int x) {
		return x & -x;
	}
	void add(int x,i64 y) {
		for(;x <= cnt;x += lowbit(x))
			c[x] = std::min(c[x] , y);
		return ;
	}
	i64 query(int x) {
		i64 ans = INF;
		for(;x;x -= lowbit(x))
			ans = std::min(ans , c[x]);
		return ans;
	}
	void clear(int x) {
		for(;x <= cnt;x += lowbit(x))
			c[x] = INF;
		return ;
	}
} tr;

void solve(int l,int r) {
	if(l >= r)
		return ;
	int mid = (l + r) >> 1;
	solve(l , mid);
	std::sort(a + l , a + mid + 1 , [&](const node& p,const node& q) {
		return (p.x2 ^ q.x2) ? p.x2 < q.x2 : p.y2 < q.y2;
	});
	int i = l;
	for(int k = mid + 1;k <= r;++ k) {
		for(;i <= mid&&a[i].x2 <= a[k].x1;++ i)
			tr.add(a[i].y2 , f[a[i].id] - c[a[i].x2] - c[a[i].y2]);
		f[a[k].id] = std::min(f[a[k].id] , c[a[k].x1] + c[a[k].y1] + tr.query(a[k].y1));
	}
	for(;i > l;-- i)
		tr.clear(a[i - 1].y2);
	solve(mid + 1 , r);
	return ;
}

int main() {
	freopen("usaco_Jan_boards.in","r",stdin);
	freopen("usaco_Jan_boards.out","w",stdout);
	scanf("%lld %d",&m,&n);
	for(int i = 1;i <= n;++ i) {
		i64 x,y,l,r;
		scanf("%lld %lld %lld %lld",&x,&y,&l,&r);
		c[++ cnt] = x;
		c[++ cnt] = y;
		c[++ cnt] = l;
		c[++ cnt] = r;
		a[i] = node(x , y , l , r);
		a[i].id = i;
	}
	c[++ cnt] = 0;
	std::sort(c + 1 , c + 1 + cnt);
	cnt = std::unique(c + 1 , c + 1 + cnt) - c - 1;
	for(int i = 0;i <= n;++ i) {
		a[i].x1 = std::lower_bound(c + 1 , c + 1 + cnt , a[i].x1) - c;
		a[i].y1 = std::lower_bound(c + 1 , c + 1 + cnt , a[i].y1) - c;
		a[i].x2 = std::lower_bound(c + 1 , c + 1 + cnt , a[i].x2) - c;
		a[i].y2 = std::lower_bound(c + 1 , c + 1 + cnt , a[i].y2) - c;
	}
	std::sort(a + 1 , a + 1 + n , [&](const node& p,const node& q) {
		return (p.x1 ^ q.x1) ? p.x1 < q.x1 : p.y1 < q.y1;
	});
	for(int i = 1;i <= n;++ i)
		f[i] = 2 * m;
	solve(0 , n);
	i64 ans = 2 * m;
	for(int i = 0;i <= n;++ i)
		ans = std::min(ans , m - c[a[i].x2] + m - c[a[i].y2] + f[a[i].id]);
	printf("%lld\n",ans);
	return 0;
}