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// Kacper Orszulak
#ifndef DEBUG
    #define NDEBUG
#endif

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vll = vector<ll>;
using vb = vector<bool>;
using vvi = vector<vi>;
using vvll = vector<vll>;
#define st first
#define nd second
#define pb push_back
#define rep(i, n) for (int i = 0; i < int(n); ++i)
#define whole(x) x.begin(), x.end()
[[maybe_unused]] constexpr int INF = 1e9+7;
template<class T> bool minR(T &a, const T &b) {
	if (b < a) { a = b; return true; }
	else return false;
}
template<class T> bool maxR(T &a, const T &b) {
	if (b > a) { a = b; return true; }
	else return false;
}

int h, w;
int n;
vi arr;

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);

    cin >> h >> w;
    cin >> n;
    arr.resize(n);
    for (int &e : arr)
        cin >> e;

    if (h % arr[0] != 0 or w % arr[0] != 0) {
        cout << "-1\n";
        return 0;
    }

    ll result = ll(h / arr[0]) * ll(w / arr[0]);

    for (int i = 1; i < n; ++i) {
        ll cntY = h / arr[i];
        ll cntX = w / arr[i];

        ll smallerCntY = cntY * ll(arr[i] / arr[i-1]);
        ll smallerCntX = cntX * ll(arr[i] / arr[i-1]);

        result -= smallerCntY * smallerCntX;

        result += cntY * cntX;
    }

    cout << result << '\n';
}