#include<fstream>
#include<list>
#include<deque>
using namespace std;
const int MaxNodesCnt = 3e4 + 5, buffSZ = 4e4 + 5;
char buff[buffSZ];
int buffPointer = buffSZ - 1;
inline void read(int &no){
    while(!isdigit(buff[buffPointer]))
        if(++buffPointer == buffSZ){
            buffPointer = 0;
            fread(buff, 1, buffSZ, stdin);
        }
    no = 0;
    while(isdigit(buff[buffPointer])){
        no = no * 10 + buff[buffPointer] - '0';
        if(++buffPointer == buffSZ){
            buffPointer = 0;
            fread(buff, 1, buffSZ, stdin);
        }
    }
}
struct Graph{
    int newNode, cost;
};
list<Graph> adjList[MaxNodesCnt];
int dist[MaxNodesCnt];
int nodesCnt, edgesCnt, startNode, endNode;
inline void readData(){
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);
    read(nodesCnt); read(edgesCnt); read(startNode); read(endNode);
    int from, to, cost;
    while(edgesCnt--){
        read(from); read(to); read(cost);
        adjList[from].push_back({to, cost});
        adjList[to].push_back({from, -cost});
    }
}
inline void BFS(int node){
    deque<int> Queue;
    Queue.push_back(node);
    int newNode, cost;
    while(!Queue.empty()){
        node = Queue.front();
        Queue.pop_front();
        for(const Graph &nextNode : adjList[node]){
            newNode = nextNode.newNode;
            cost = nextNode.cost;
            if(dist[newNode] == 0 and newNode != startNode){
                dist[newNode] = dist[node] + cost;
                Queue.push_back(newNode);
            }
        }
    }
}
int main(){
    readData();
    BFS(startNode);
    printf("%d", dist[endNode]);
}