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#include <iostream>
#define int long long
using namespace std;
int n, t[35];

int f(int a, int b, int i) {
    if (a == 0 || b == 0)
        return 0;
    for (; i >= 0; i--)
        if (t[i] <= a && t[i] <= b)
            return (a / t[i]) * (b / t[i]) + f(a - a % t[i], b % t[i], i - 1) + f(a % t[i], b, i - 1);
    cout << -1;
    exit(0);
}

int32_t main() {
    int h, w;
    cin >> h >> w >> n;
    for (int i = 0; i < n; i++)
        cin >> t[i];
    cout << f(h, w, n - 1);
}