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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;

template<class T> using ordered_set =tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update> ;

#define debug(x) cout << "[" <<  #x << " " << x << "] ";

#define ar array
#define ll long long
#define ld long double
#define sz(x) ((int)x.size())
#define all(a) (a).begin(), (a).end()

typedef vector<int> vi;
typedef pair<int,int> pi;

const int MAX_N = 1e5 + 5;
const ll MOD = 1e9 + 7;
const ll INF = 1e9;
const ld EPS = 1e-9;

void solve() {
    ll h,w,n;
    cin >> h >> w >> n;
    vector<ll> a(n);
    for(auto& x : a)cin >> x;

    if(h%a[0] || w%a[0]){
        cout << -1 << "\n";
        return;
    }
    h = h/a[0];
    w = w/a[0];
    ll res = h*w;
    for(int i=1; i<n; i++){
        ll relsize = a[i]/a[i-1];
        h = h/relsize;
        w = w/relsize;
        res -= h*w*(relsize*relsize-1);
    }
    cout << res << "\n";
}


int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    int tc = 1;
    for (int t = 1; t <= tc; t++) {
        // cout << "Case #" << t << ": ";
        solve();
    }
}