LCT
a排序后做b的kruskal
splay维护最大边的位置，若当前大，不加，否则cut，加边
# include <stdio.h>
# include <stdlib.h>
# include <iostream>
# include <algorithm>
# include <string.h>
# define IL inline
# define RG register
# define Fill(a, b) memset(a, b, sizeof(a))
using namespace std;
typedef long long ll;
IL ll Read(){
RG char c = getchar(); RG ll x = 0, z = 1;
for(; c < '0' || c > '9'; c = getchar()) z = c == '-' ? -1 : 1;
for(; c >= '0' && c <= '9'; c = getchar()) x = (x << 1) + (x << 3) + c - '0';
return x * z;
}
const int MAXN(2e5 + 10);
int S[MAXN], n, m, ans = 1 << 30;
struct Edge{
int u, v, a, b;
IL bool operator <(RG Edge B) const{ return a < B.a; }
} edge[MAXN];
struct Tree{ int fa, ch[2], rev, maxb; } t[MAXN];
IL bool Isroot(RG int x){ return t[t[x].fa].ch[0] != x && t[t[x].fa].ch[1] != x; }
IL void Update(RG int x){
t[x].maxb = x;
if(edge[t[t[x].ch[0]].maxb].b > edge[t[x].maxb].b) t[x].maxb = t[t[x].ch[0]].maxb;
if(edge[t[t[x].ch[1]].maxb].b > edge[t[x].maxb].b) t[x].maxb = t[t[x].ch[1]].maxb;
}
IL void Pushdown(RG int x){
if(!t[x].rev) return;
swap(t[x].ch[0], t[x].ch[1]);
t[x].rev = 0; t[t[x].ch[0]].rev ^= 1; t[t[x].ch[1]].rev ^= 1;
}
IL bool Son(RG int x){ return t[t[x].fa].ch[1] == x; }
IL void Rot(RG int x){
RG int y = t[x].fa, z = t[y].fa, c = Son(x);
if(!Isroot(y)) t[z].ch[Son(y)] = x; t[x].fa = z;
t[y].ch[c] = t[x].ch[!c]; t[t[y].ch[c]].fa = y;
t[x].ch[!c] = y; t[y].fa = x;
Update(y);
}
IL void Splay(RG int x){
RG int top = 0; S[++top] = x;
for(RG int y = x; !Isroot(y); y = t[y].fa) S[++top] = t[y].fa;
while(top) Pushdown(S[top--]);
for(RG int y = t[x].fa; !Isroot(x); Rot(x), y = t[x].fa)
if(!Isroot(y)) Son(x) ^ Son(y) ? Rot(x) : Rot(y);
Update(x);
}
IL void Access(RG int x){ for(RG int y = 0; x; y = x, x = t[x].fa) Splay(x), t[x].ch[1] = y, Update(x); }
IL void Makeroot(RG int x){ Access(x); Splay(x); t[x].rev ^= 1; }
IL int Findroot(RG int x){ Access(x); Splay(x); while(t[x].ch[0]) x = t[x].ch[0]; return x; }
IL void Split(RG int x, RG int y){ Makeroot(x); Access(y); Splay(y); }
IL void Link(RG int x, RG int y){ Makeroot(x); t[x].fa = y; }
IL void Cut(RG int x, RG int y){ Split(x, y); t[x].fa = t[y].ch[0] = 0; }
IL int Query(RG int u, RG int v){ Split(u, v); return t[v].maxb; }
int main(RG int argc, RG char* argv[]){
n = Read(); m = Read();
for(RG int i = 1; i <= m; i++) edge[i] = (Edge){Read(), Read(), Read(), Read()};
sort(edge + 1, edge + m + 1);
for(RG int i = 1; i <= m; i++){
RG int u = edge[i].u + m, v = edge[i].v + m;
if(Findroot(u) != Findroot(v)) Link(u, i), Link(v, i);
else{
RG int mx = Query(u, v);
if(edge[i].b < edge[mx].b) Cut(edge[mx].u + m, mx), Cut(edge[mx].v + m, mx), Link(u, i), Link(v, i);
}
if(Findroot(1 + m) == Findroot(n + m)) ans = min(ans, edge[i].a + edge[Query(1 + m, n + m)].b);
}
printf("%d
", ans != (1 << 30) ? ans : -1);
return 0;
}