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#include <bits/stdc++.h>
using namespace std;
#define rep(a, b) for (int a = 0; a < (b); a++)
#define rep1(a, b) for (int a = 1; a <= (b); a++)
#define all(x) (x).begin(), (x).end()
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int MOD = 1e9 + 7;

#define LOCAL false

int sh, sw;
int n;
int sizes[69];

ll get(int h, int w) {
    if (h > w) swap(h, w);
    if (h == 0) return 0;
    int s = -1;
    for (int i = n-1; i >= 0; i--) if (sizes[i] <= h) {
        s = sizes[i];
        break;
    }
    if (s == -1) {
        cout << "-1\n";
        exit(0);
    }
    int hc = h/s;
    int wc = w/s;
    return (ll)hc*wc + get(h-hc*s, wc*s) + get(h, w%s);
}

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
    if (LOCAL) {
        ignore=freopen("io/in", "r", stdin);
        ignore=freopen("io/out", "w", stdout);
    }

    cin >> sh >> sw;
    cin >> n;
    rep(i, n) cin >> sizes[i];

    cout << get(sh, sw) << '\n';

    return 0;
}