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#include <bits/stdc++.h>
#define debug(x...) cerr << "[" #x "]: ", [](auto... $) {((cerr << $ << "; "), ...); }(x), cerr << '\n'
using namespace std;
typedef long long ll;

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

    ll h, w, n;
    cin >> h >> w >> n;

    vector<ll> a(n);
    for(int i = 0; i < n; i++){
        cin >> a[i];
    }

    sort(a.begin(), a.end(), greater<ll>());

    ll obrazy = 0;
    ll wym = h * w;
    ll i = 0;

    while(wym > 0 && i < n){
        ll okok = wym / (a[i] * a[i]);
        wym -= okok * a[i] * a[i];
        obrazy += okok;
        i++;
    }
    if(wym == 0){
        cout << obrazy << '\n';
    } else {
        cout << -1 << '\n';
    }

    return 0;
}