C++ :

#include <bits/stdc++.h>

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  int n, m, a, b;
  std::cin >> n >> m >> a >> b;
  std::vector<int> v(n);
  for (auto &i : v) std::cin >> i;
  std::vector<std::vector<int>> chg(n);
  for (int x, y; m; --m) {
    std::cin >> x >> y;
    chg[x].push_back(y);
  }
  std::vector<int> f(n, 1100000000);
  f[a] = 0;
  std::queue<int> Q;
  for (Q.push(a); !Q.empty(); Q.pop()) {
    int u = Q.front();
    for (auto v : chg[u]) {
      if (f[v] > f[u] + 1) {
        f[v] = f[u] + 1;
        Q.push(v);
      }
    }
  }
  if (f[b] == 1100000000) {
    std::cout << "No solution\n";
  } else {
    std::cout << f[b] + v[b] - v[a] << std::endl;
  }
}