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#include <cstdio>
#include <vector>

using namespace std;

unsigned long long get_result(vector<unsigned long long>& d, unsigned long long h, unsigned long long w) {
  if (h == 0 || w == 0) {
    return 0;
  }

  for (int i = d.size() - 1; i >= 0; i--) {
    unsigned long long w_fit = w / d[i];
    unsigned long long h_fit = h / d[i];
    unsigned long long paintings = w_fit * h_fit;

    if (paintings) {
      return paintings + get_result(d, h - h_fit * d[i], w_fit * d[i]) + get_result(d, h_fit * d[i], w - w_fit * d[i]) + get_result(d, h - h_fit * d[i], w - w_fit * d[i]);
    }
  }

  return 0;
}

int main() {
  unsigned long long h, w;

  scanf("%llu %llu", &h, &w);

  int n;

  scanf("%d", &n);

  vector<unsigned long long> d(n);

  for (int i = 0; i < n; i++) {
    scanf("%llu", &d[i]);
  }

  if (h % d[0] != 0 || w % d[0] != 0) {
    puts("-1");
  }
  else {
    printf("%llu\n", get_result(d, h, w));
  }

  return 0;
}