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#include <bits/stdc++.h>
using namespace std;

#define pb push_back
#define fi first
#define sn second

typedef long long ll;
typedef vector<ll> VI;
typedef vector<char> VC;
typedef pair<int, int> PI;

ll solve(ll h, ll w, int n, VI &A) {
  if (w < A[n - 1] || h < A[n - 1]) return -1;
  if (w % A[n - 1] != 0 || h % A[n - 1] != 0) return -1;

  ll r = 0;
  ll h0 = h;
  for (int i0 = 0; i0 < n; i0++) {
    ll a0 = A[i0];

    if (a0 > h0) continue;

    ll r0 = 0;
    ll w0 = w;

    for (int i1 = i0; i1 < n; i1++) {
      ll a1 = A[i1];

      r0 += (w0 / a1) * (a0 / a1);
      w0 %= a1;
    }

    r += (h0 / a0) * r0;
    h0 %= a0;
  }

  return r;
}

int main() {
  ll h, w;
  cin >> h >> w;
  int n;
  cin >> n;

  VI A(n);
  for (int i = n - 1; i >= 0; i--) {
    cin >> A[i];
  }

  cout << solve(h, w, n, A) << endl;

  return 0;
}