我是真的没有想到这是一道板子题QvQ:
/*
为什么感觉这是最短路的板子题啊
*/
#include <queue>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define ll long long
#define rep(i,x,y) for(ll i=(x);i<=(y);i++)
#define repl(i,x,y) for(ll i=(x);i<(y);i++)
#define repd(i,x,y) for(ll i=(x);i>=(y);i--)
using namespace std;
const ll N=1e4+5;
const ll Inf=1e18;
ll n,m,st,ed,dis[N],vis[N];
ll cnt,to[N<<1],nxt[N<<1],edge[N<<1],head[N];
inline ll read() {
ll x=0;char ch=getchar();bool f=0;
while(ch>'9'||ch<'0'){if(ch=='-')f=1;ch=getchar();}
while(ch>='0'&&ch<='9'){x=(x<<1)+(x<<3)+ch-'0';ch=getchar();}
return f?-x:x;
}
void ins(ll x,ll y,ll z) {
to[++cnt]=y;edge[cnt]=z;nxt[cnt]=head[x];head[x]=cnt;
}
void spfa() {
rep(i,1,n) dis[i]=Inf;
queue<ll>q;q.push(st);dis[st]=0;vis[st]=1;
while(q.size()) {
ll x=q.front();q.pop();vis[x]=0;
for(ll i=head[x];i;i=nxt[i]) {
ll y=to[i],z=edge[i];
if(dis[y]>dis[x]+z) {
dis[y]=dis[x]+z;
if(!vis[y]) vis[y]=1,q.push(y);
}
}
}
}
int main() {
n=read(),m=read();
st=read(),ed=read();
rep(i,1,m) {
ll x=read(),y=read(),z=read();
ins(x,y,z);ins(y,x,z);
}
spfa();
printf("%lld\n",dis[ed]);
return 0;
}