#include <cstdio>
#include <cmath>
#include <vector>
#include <algorithm>
#include <cassert>
#include <cstring>
#include <climits>
#include <queue>
#include <stack>
using namespace std;
typedef long long LL;
typedef unsigned long long hsh;

#ifdef INFOARENA
#define ProblemName "sate"
#endif

#define MCONCAT(A, B) A B
#ifdef ProblemName
#define InFile MCONCAT(ProblemName, ".in")
#define OuFile MCONCAT(ProblemName, ".out")
#else
#define InFile "fis.in"
#define OuFile "fis.out"
#endif

const int MAXN = 30010;
vector< pair<int, int> > G[MAXN];
int dst[MAXN];
int S[MAXN], ssz;

void DFS(int nod, int tar) {
  ssz = 0;
  S[ssz++] = nod;
  dst[nod] = 0;
  while(ssz) {
    int x = S[--ssz];
    for (const auto &it : G[x]) {
      if (dst[it.first] >= 0)
        continue;
      dst[it.first] = dst[x] + it.second;
      if (it.first == Y)
        return;
      S[ssz++] = it.first;
    }
  }
}

int main() {
  assert(freopen(InFile, "r", stdin));
  assert(freopen(OuFile, "w", stdout));
  memset(dst, 0xFF, sizeof(dst));
  int N, M, X, Y;
  scanf("%d%d%d%d", &N, &M, &X, &Y);
  --X, --Y;
  while (M--) {
    int a, b, c;
    scanf("%d%d%d", &a, &b, &c);
    --a, --b;
    if (a > b)
      swap(a, b);
    G[a].push_back(make_pair(b, c));
    G[b].push_back(make_pair(a, -c));
  }
  DFS(X, Y);
  printf("%d\n", dst[Y]);
  return 0;
}