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

int main(){
    ios_base::sync_with_stdio(0);
    cout.tie(0); cin.tie(0);
    ll h, w, n, answer; cin>>h>>w>>n;
    //---------------------
    ll d; cin>>d;
    if(h % d != 0 || w % d != 0){cout<<"-1"; return 0;}
    answer = (h/d) * (w/d);
    //---------------------
    n--; while(n--){
        ll now; cin>>now;
        ll now_teil = ((ll)(h/now)) * ((ll)(w/now));
        answer -= ((now/d) * (now/d) - 1) * now_teil;
        h -= h % now; w -= w % now;
        d = now;
    }
    //---------------------
    cout<<answer;
    return 0;
}