#include <bits/stdc++.h>

using namespace std;

#ifdef D
#define DEBUG(x)        \
    do {                \
        x               \
        cout.flush();   \
    } while (0)
#else
#define DEBUG(x)
#endif

using ll = long long;
using pii = pair<int, int>;

const int NMAX = 3e4 + 7;

int w, h, n;
int d[37];

ll solve(int w, int h, int k) {
    if (w == 0 || h == 0) {
        return 0;
    }
    ll cw = 0, ch = 0;
    while (cw == 0 || ch == 0) {
        cw = w / d[k];
        ch = h / d[k];
        k++;
    }
    k--;
    ll res = cw * ch;
    res += solve(w - cw * d[k], h, k + 1);
    res += solve(cw * d[k], h - ch * d[k], k + 1);
    return res;
}

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

    cin >> h >> w >> n;

    for (int i = 0; i < n; ++i) {
        cin >> d[i];
    }

    sort(d, d + n, greater<>());

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

    cout << solve(w, h, 0) << "\n";

    return 0;
}