#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define per(i, a, b) for (int i = b - 1; i >= (a); --i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
#define sz(x) x.size()
#define pb push_back
#define umap unordered_map
#define uset unordered_set
typedef pair<int, int> ii;
typedef pair<int, ii> iii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<vi> vvi;
typedef long long ll;
const int INF = 1000000007;
vii graph[50001];
int main() {
cin.tie(0)->sync_with_stdio(0);
cin.exceptions(cin.failbit);
int k, n, m, o, dist[50001];
cin >> k >> n >> m >> o;
rep(i, 0, m) {
int u, v, w;
cin >> u >> v >> w;
graph[u].emplace_back(v, w);
}
set<ii> pq;
rep(i, 0, o) {
fill(dist, dist + n, INF);
int a, b;
cin >> a >> b;
dist[a] = 0;
rep(i, 0, n) pq.insert({dist[i], i});
while (!pq.empty()) {
int d,u;
tie(d, u) = *pq.begin();
pq.erase(pq.begin());
trav(edge, graph[u]) {
int v, w;
tie(v, w) = edge;
if (d+w>=dist[v])continue;
pq.erase({dist[v],v});
dist[v]=d+w;
pq.emplace(d+w,v);
}
}
if(dist[b]==INF)cout<<"-1\n";
else cout<<dist[b]<<endl;
}
return 0;
}
