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

long long solve(vector <long long> &v, int idx, long long w, long long h) {
    while (v[idx] > w || v[idx] > h) idx--;
    long long x = w / v[idx], y = h / v[idx];
    long long _w = w - x * v[idx], _h = h - y * v[idx];

    if (_w == 0 && _h == 0) return y * x;
    else if (_w == 0) return y * x + solve(v, idx - 1, w, _h);
    else if (_h == 0) return y * x + solve(v, idx - 1, _w, h);
    else return y * x + solve(v, idx - 1, w - _w, _h) + solve(v, idx - 1, _w, h - _h) + solve(v, idx - 1, _w, _h);
}

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    long long h, w;
    int n;
    cin >> h >> w >> n;
    vector <long long> v(n + 1);
    for (int i = 1; i <= n; i++)
        cin >> v[i];

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

    cout << solve(v, n, w, h) << '\n';
}