#include <bits/stdc++.h>
using namespace std;
#define int long long
#define f first
#define s second
#define endl '\n'
#define all(x) begin(x), end(x)
int n, ca, cb, k;
int u, v, l;
int ans = INT_MAX;
vector<tuple<int,int,int>> va, vb;
vector<pair<int,pair<int,int>>> lga[200069], lgb[200069];
int cnt, cnta[200069], cntb[200069];
vector<int> chd[200069];
int par[200069];
pair<int,int> rts[200069];
map<int,int> mp[200069];
void dsuclr() {
for (int i = 0; i < n; i++) {
par[i] = i;
chd[i].clear();
chd[i].push_back(i);
}
cnt = 0;
}
void merge(int x, int y, vector<pair<int,pair<int,int>>> &lg) {
x = par[x]; y = par[y];
if (x == y) return;
if (chd[x].size() < chd[y].size()) swap(x, y);
cnt += chd[x].size() * chd[y].size();
for (auto i : chd[y]) {
par[i] = x;
chd[x].push_back(i);
lg.push_back({i, {y, x}});
}
chd[y].clear();
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cin>>n>>ca>>cb>>k;
dsuclr();
for (int i = 0; i < ca; i++) {
cin>>u>>v>>l; u--; v--;
va.push_back({l, u, v});
}
sort(all(va));
for (int i = 0; i < ca; i++) {
tie(l, u, v) = va[i];
merge(u, v, lga[i + 1]);
cnta[i + 1] = cnt;
}
for (int i = 0; i < n; i++) {
rts[i] = {par[i], i};
mp[par[i]][i] = 1;
}
dsuclr();
for (int i = 0; i < cb; i++) {
cin>>u>>v>>l; u--; v--;
vb.push_back({l, u, v});
}
sort(all(vb));
for (int i = 0; i < cb; i++) {
tie(l, u, v) = vb[i];
merge(u, v, lgb[i + 1]);
cntb[i + 1] = cnt;
}
cnt = 0;
int tb = 0;
for (int ta = ca; ~ta; ta--) {
for (auto t : lga[ta + 1]) {
int i = t.f;
mp[rts[i].f][rts[i].s]--;
cnt-=mp[rts[i].f][rts[i].s];
rts[i].f = t.s.f;
cnt+=mp[rts[i].f][rts[i].s];
mp[rts[i].f][rts[i].s]++;
}
while (tb < cb && cnta[ta] + cntb[tb] - cnt < k) {
for (auto t : lgb[tb + 1]) {
int i = t.f;
mp[rts[i].f][rts[i].s]--;
cnt-=mp[rts[i].f][rts[i].s];
rts[i].s = t.s.s;
cnt+=mp[rts[i].f][rts[i].s];
mp[rts[i].f][rts[i].s]++;
}
tb++;
}
if (cnta[ta] + cntb[tb] - cnt >= k) {
int tmpa = (ta == 0 ? 0 : get<0>(va[ta - 1]));
int tmpb = (tb == 0 ? 0 : get<0>(vb[tb - 1]));
ans = min(ans, tmpa + tmpb);
}
}
cout<<(ans == INT_MAX ? -1 : ans)<<endl;
return 0;
}
