#include <bits/stdc++.h>

#ifdef DEBUG
#include <Mylib/Debug/debug.cpp>
#else
#define dump(...)
#endif


int main() {
  int N, M; std::cin >> N >> M;
  std::vector<int> A(N), B(M);

  for (int i = 0; i < N; ++i) std::cin >> A[i];
  for (int i = 0; i < M; ++i) std::cin >> B[i];

  std::vector<int> a, b;
  for (int i = 0; i < M; ++i) a.insert(a.end(), A.begin(), A.end());
  for (int i = 0; i < N; ++i) b.insert(b.end(), B.begin(), B.end());

  dump(a, b);

  int ans = INT_MAX;
  bool ok = false;

  for (int i = 0; i < N * M; ++i) {
    if (a[i] == b[i]) {
      ok = true;
      ans = std::min(ans, i + 1);
    }
  }

  if (ok) std::cout << ans << "\n";
  else std::cout << -1 << "\n";

  return 0;
}