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#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include "debug.h"
#else
#define debug(...)
#endif

#define x first
#define y second
#define ir(a, x, b) ((a) <= (x) && (x) <= (b))
#define vec vector
#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define all(x) (x).begin(), (x).end()
#define endl "\n";

using ll = long long;

int main() {
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    ll h, w;
    cin >> h >> w;
    ll n;
    cin >> n;
    ll last;
    cin >> last;
    if (h % last != 0 || w % last != 0) {
        cout << "-1\n";
        return 0;
    }
    ll s = 0;
    rep (i, 1, n) {
        ll c; cin >> c;
        ll nh = (h / c) * c, nw = (w / c) * c;
        s += (h*w - nh*nw)/(last*last);
        last = c, h = nh, w = nw;
    }
    s += h*w/(last*last);
    cout << s << "\n";
    return 0;
}