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

signed main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int h, w, mw, mh;
    cin >> h >> w;
    mw = w, mh = h;
    int n, res =0;
    cin >> n;
    vector<int> inp(n+1);
    inp[0] = 1;
    for(int i=0; i<n; ++i) cin >> inp[i+1];

    if(w%inp[1] >0 || h%inp[1] > 0){
        cout << -1;
        return 0;
    }


    int il = 1;
    for(int i=1, a, b; i<n; ++i){
        w/= inp[i]/inp[i-1], h/=inp[i]/inp[i-1];
        a = h%(inp[i+1]/inp[i]);
        b = w%(inp[i+1]/inp[i]);
        res += h*b + w*a - a*b;
        il = inp[i];
        // cout << h << " " << w << " " << a << " " << b << " ";
        // cout << res << "\n";
    }
    w/= inp[n]/inp[n-1], h/=inp[n]/inp[n-1];

    // cout << h << " " << w << "\n";
    cout << res  + (h*w) << "\n";
}