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#ifdef DEBUG
#define _GLIBCXX_DEBUG
#endif
//#pragma GCC optimize("O3")
#include<bits/stdc++.h>
using namespace std;

#define pb push_back

typedef long long ll;
typedef long double ld;
ll h, w;
ll d[35];
int n;
ll solve(ll h, ll w) {
    if (h == 0 || w == 0) return 0;
    if (h < w) swap(h, w);
    for (int i = n - 1; i >= 0; i--) {
        if (h >= d[i] && w >= d[i]) {
            ll S = (h / d[i]) * (w / d[i]) + solve(h % d[i], w % d[i]) + (h / d[i]) * solve(d[i], w % d[i]) + (w / d[i]) * solve(d[i], h % d[i]);
            return S;
        }
    }
    assert(false);
    return -228;
}
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
#ifdef DEBUG
    freopen("input.txt", "r", stdin);
#endif
    cin >> h >> w;
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> d[i];
    }
    if (h % d[0] != 0 || w % d[0] != 0) {
        cout << -1;
        return 0;
    }
    cout << solve(h, w);
    return 0;
}