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#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
typedef long long ll;
typedef pair<ll,ll> pll;
typedef pair<ll,int> pli;
typedef pair<int,ll> pil;
typedef pair<int,int> pii;
const ll INFLL=1e18+7;
const int INF=1e9+7;
#define pb push_back
int main()
{
	ios_base::sync_with_stdio(0);
    ll h,w; cin>>h>>w;
    int n; cin>>n;
    vector<ll> lens(n);
    for(int i=0;i<n;++i) cin>>lens[i];
    reverse(lens.begin(), lens.end());
    vector<ll> H(n), W(n);
    // h
    ll left = h;
    for(int i=0;i<n;++i){
        int x = lens[i];
        H[i] = left/x;
        left %= x;
    }
    if(left){
        cout<<"-1\n";
        return 0;
    }
    // w
    left = w;
    for(int i=0;i<n;++i){
        int x = lens[i];
        W[i] = left/x;
        left %= x;
    }
    if(left){
        cout<<"-1\n";
        return 0;
    }
    ll ans = 0;
    for(int i=n-1;i>=0;--i){
        ans += W[i]*(h/lens[i]) + - W[i]*H[i] + H[i]*(w/lens[i]);
        h -= H[i]*lens[i];
        w -= W[i]*lens[i];
    }
    cout<<ans<<"\n";
}