#include <bits/stdc++.h>
using namespace std;

long long h, w;

int n;
long long obr[30];

long long int res;

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    cin >> h >> w;
    cin >> n;
    for(int i = 0; i < n; i++){
        cin >> obr[i];
    }

    if(h % obr[0] != 0 || w % obr[0] != 0){
        cout << -1 << "\n";
        return 0;
    }

    if(h > w) swap(h, w);

    for(int i = 0; i < n -1; i++){
        long long x = h % obr[i+1];
        long long rem = x;
        x /= obr[i];

        long long y = w / obr[i];

        res += x * y;

        h -= rem;

        x = w % obr[i+1];
        rem = x;
        x /= obr[i];

        y = h / obr[i];

        res += x * y;

        w -= rem;
    }

    res += (h / obr[n-1]) * (w / obr[n-1]);

    cout << res << "\n";
}