#include <cstdio>
#include <vector>
#include <queue>

using namespace std;

#define Nmax 30100

vector<int> l[Nmax],c[Nmax];
queue<int> q;
char v[Nmax];

int ret[Nmax];

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

        int n,m,x,y,a,b,C;

        scanf("%d%d%d%d",&n,&m,&x,&y);

        for (int i=1;i<=m;++i)
        {
                scanf("%d%d%d",&a,&b,&C);
                l[a].push_back(b); c[a].push_back(C);
                l[b].push_back(a); c[b].push_back(-C);
        }

        if(x > y) x^=y^=x^=y;
        
	q.push(x);
	v[x] = 1;
	int nod;

	while(!q.empty())
	{
		nod = q.front(); q.pop();
		if(nod == y) break;
        	for (int i=0;i<l[nod].size();++i)
		if(v[l[nod][i]] == 0)
      		{
	    		ret[l[nod][i]] = ret[nod] + c[nod][i];
	    		v[l[nod][i]] = 1;
			q.push(l[nod][i]);
		}
	}
        
        printf("%d\n",ret[y]);

        return 0;
}