#include<algorithm>

#include<cstring>

#include<cstdio>

#include<iostream>

using namespace std;

const int N=1e3+;

const int M=3e3+;

const int inf=1e6;

int n,m,tot;

int head[N],L[M],vis[N];

struct EDGE{

    int to,nxt,l,r;

}edge[M<<];

inline int read(){

    char ch=getchar();int s=,f=;

    while (ch<''||ch>'') {if (ch=='-') f=-;ch=getchar();}

    while (ch>=''&&ch<='') {s=(s<<)+(s<<)+ch-'';ch=getchar();}

    return s*f;

}

void link(int u,int v,int l,int r){

    edge[++tot]=(EDGE){v,head[u],l,r};

    head[u]=tot;

}

bool dfs(int x,int l,int r)

{

    if (x==n) return ;

    if (vis[x]) return ;

    vis[x]=;

    for (int i=head[x];i;i=edge[i].nxt)

    {

        if (edge[i].l<=l&&edge[i].r>=r)

        {

            if (dfs(edge[i].to,l,r)) return ;

        }

    }

    return ;

}

bool check(int l,int r)

{

    memset(vis,,sizeof(vis));

    if (dfs(,l,r)) return ;

    return ;

}

int main()

{

    n=read();m=read();

    for (int i=,u,v,l,r;i<=m;i++){

        u=read();v=read();l=read();r=read();

        link(u,v,l,r);link(v,u,l,r);

        L[i]=l;

    }

    sort(L+,L++m);

    int ans=,al,ar;

    for (int i=m;i;i--){

        int l=L[i],r=inf;

        while (l+<r)

        {

            int mid=l+r>>;

            if (check(L[i],mid))

            {

                if (mid-L[i]+>=ans)

                {

                    ans=mid-L[i]+;

                    al=L[i];ar=mid;

                }

                l=mid;

            }

            else r=mid;

        }

    }

    printf("%d\n",ans);

    for (int i=al;i<=ar;i++) printf("%d ",i);

    return ;

}