#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
using ii = pair<int, int>;
using ll = long long;
using db = long double;
const int N = 305 + 5, M = 1e7 + 5, K = 20, MOD = 1e9 + 7, MOD2 = 998244353;
const ll MX = 1e15;
const int blkSz = 320;
int mat[N][N];
int vis[N], a[N], b[N], n, cap, e;
int cas = 0;
vector<int> par;
vector<int> from, to, values;
void addToAns(int u, int v, int w) {
  from.push_back(u);
  to.push_back(v);
  values.push_back(w);
  a[v] += w;
  a[u] -= w;
}
void raise(int u, int val) {
  if (!par[u]) return;
  if (a[par[u]] + val > cap) {
    raise(par[u], val), addToAns(u, par[u], val);
  } else {
    addToAns(u, par[u], val), raise(par[u], val);
  }
}
void solve(int cur) {
  queue<int> q;
  q.push(cur);
  vis[cur] = ++cas;
  vector<int> nodes;
  par.resize(n + 1);
  while (q.size()) {
    int u = q.front();
    q.pop();
    nodes.push_back(u);
    for (int v = 1; v <= n; v++)
      if (mat[u][v]) {
        if (vis[v] != cas) {
          vis[v] = cas;
          q.push(v);
          par[v] = u;
        }
      }
  }
  par[cur] = 0;
  for (int i = 1; i < nodes.size() && a[cur] != b[cur]; i++) {
    int u = nodes[i];
    if (a[u] > b[u]) {
      int x = min(b[cur] - a[cur], a[u] - b[u]);
      if (x > cap / 2) {
        raise(u, cap / 2);
        x -= cap / 2;
      }
      raise(u, x);
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n >> cap >> e;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 1; i <= n; i++) cin >> b[i];
  for (int i = 0, u, v; i < e; i++) {
    cin >> u >> v;
    mat[u][v] = mat[v][u] = 1;
  }
  for (int i = 1; i <= n; i++) {
    if (a[i] < b[i]) solve(i);
  }
  for (int i = 1; i <= n; i++) {
    if (a[i] != b[i]) {
      cout << "NO\n";
      return 0;
    }
  }
  cout << from.size() << '\n';
  for (int i = 0; i < from.size(); i++) {
    cout << from[i] << ' ' << to[i] << ' ' << values[i] << '\n';
  }
  return 0;
}
