#include <cstdio>
#include <vector>
#include <bitset>
#include <cstring>
#include <queue>

#define Nmax 30005
#define INF 0x3f3f3f3f

using namespace std;
int N,M,X,Y;
vector<pair<int,int> > G[Nmax];
queue<int> Q;
bitset<Nmax> used;

int DP[Nmax];

void read()
{
    scanf("%d%d%d%d",&N,&M,&X,&Y);
    int a,b,c;
    for(int i = 1; i <= M; ++i)
    {
        scanf("%d%d%d",&a,&b,&c);
        G[a].push_back(make_pair(c,b));
        G[b].push_back(make_pair(c,a));
    }
}

void solve()
{
    ///memset(DP,INF,sizeof(DP));
    DP[X] = 0;
    used[X] = 1;
    Q.push(X);
    int k;
    while(!Q.empty())
    {
        k = Q.front(); Q.pop();
        used[k] = 1;
        for(vector<pair<int,int> >::iterator it = G[k].begin(); it != G[k].end(); ++it)
            if(!used[it->second])
            {
                if(it->second > k)
                    DP[it->second] = DP[k] + it->first;
                else
                    DP[it->second] = DP[k] - it->first;
                Q.push(it->second);
                if(DP[Y])
                {
                    printf("%d\n",DP[Y]);
                    return;
                }
            }
    }
    printf("%d\n",DP[Y]);
}

int main()
{
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);

    read();
    solve();

    return 0;
}