#include<iostream>
#include<cstdio>
#include<queue>
#include<vector>
using namespace std;
queue < int > q;
vector < pair < int, int > > L[30010];
int cost[30010],x,y,c,n,m,ic,sf,i;
int main()
{
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    scanf("%d%d%d%d",&n,&m,&ic,&sf);
    while(m--)
    {
        scanf("%d%d%d",&x,&y,&c);
        L[x].push_back(make_pair(y,c));
        L[y].push_back(make_pair(x,-c));
    }
    q.push(ic); cost[ic]=1;
    while(!q.empty())
    {
        x=q.front();
        for(i=0; i<L[x].size(); ++i)
            if(cost[L[x][i].first]==0)
            {
                cost[L[x][i].first]=cost[x]+L[x][i].second;
                if(L[x][i].first==sf)
                {
                    printf("%d\n",cost[sf]-1);
                    return 0;
                }
                q.push(L[x][i].first);
            }
        q.pop();
    }
    return 0;
}