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#include <ios>
#include <iostream>
#include <ostream>
#include <vector>
#include <algorithm>
using namespace std;

int n;


int wypelnijOdDolu (int h, int w, const vector <int> &D, int it_beg) {
    int sum= 0;

    for (int k=it_beg; k>=0; k--) {
        int can_on_y= h / D[k];
        if (can_on_y == 0)
            continue;

        sum+= can_on_y * (w / D[k]);
        h%= D[k];
        if (h == 0)
            break;
    }

    if (h != 0)
        return -1;

    return sum;
}




int main () {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int h, w;
    cin >> h >> w;

    cin >> n;

    vector <int> D(n);
    for (int i=0; i<n; i++)
        cin >> D[i];

    int sum= 0;
    int new_w= w;
    for (int k=n-1; k>=0; k--) {
        int can_on_x= new_w / D[k];
        int can_on_y= h / D[k];

        if (can_on_x * can_on_y == 0)
            continue;

        sum+= can_on_x * can_on_y;
        new_w%= D[k];

        if (h % D[k] != 0) {
            int temp= wypelnijOdDolu(can_on_y * D[k], can_on_x * D[k], D, k-1);
            if (temp == -1) {
                cout << -1 << endl;
                return 0;
            }
            sum+= temp;
        }

        if (new_w == 0)
            break;
    }

    if (new_w != 0) {
        cout << -1 << endl;
        return 0;
    }

    cout << sum << endl;
    return 0;
}