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

vector <int> a;
int ans;

int32_t main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    ans = 0;
    int h, w, n;
    cin >> h >> w >> n;
    a.resize(n);
    for(int i = 0; i < n; i++)
        cin >> a[i];
    sort(a.begin(), a.end());
    if(h%a[0]+w%a[0] > 0)
    {
        cout << -1;
        return 0;
    }
    reverse(a.begin(), a.end());
    int lasts = 0;
    for(auto x: a){
        ans += (h/x)*(w/x);
        ans -= lasts/(x*x);
        lasts = (h/x)*(w/x)*x*x;
    }
    cout << ans;
    return 0;
}