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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/rope>

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int uint;

typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef vector<long long> VL;
typedef vector<vector<long long>> VVL;

typedef vector<char> VC;
typedef vector<vector<char>> VVC;
typedef vector<bool> VB;
typedef vector<vector<bool>> VVB;

typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<pair<int, int>> VPI;
typedef vector<pair<long long, long long>> VPL;

#define LINE '\n'
#define SPACE ' '
#define PB push_back
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define ALL(x) x.begin(), x.end()
#define RALL(x) x.rbegin(), x.rend()

// count, max_width
pair<ll, ll> compute(VL vec, ll h, ll w) {
    ll left = h;
    ll cur_max_width = 0;
    ll ans = 0;
    for(int i = vec.size() - 1; i >= 0; i--) {
        if(vec[i] > w) continue;

        // ile razy wyzej zmiesci sie vec[i]
        ll times = (left / vec[i]);
        left -= times * vec[i];
        if(cur_max_width != 0) {
            // mamy times w gore i cc/ve[i] w bok
            ans += times * (cur_max_width / vec[i]);
        }
        else {
            ans += times;
            cur_max_width = vec[i];
        }

        if(left == 0) break;
    }
    //cerr << "H: " << h << "W: " << w << LINE;
    //cerr << ans << SPACE << cur_max_width << LINE << LINE;
    return {ans, cur_max_width};
}

void solve()
{
    ll h, w;
    cin >> h >> w;

    int n;
    cin >> n;
    vector<ll> vec(n);
    FOR(i, 0, n) {
        ll a;
        cin >> a;
        vec[i] = a;
    }

    if(h % vec[0] != 0 || w % vec[0] != 0) {
        cout << -1 << LINE;
        return;
    }
    
    ll left = w;
    ll ans = 0;
    while(left > 0) {
        PLL comp = compute(vec, h, left);
        ll times = left / comp.second;
        ans += times * comp.first;
        left -= times * comp.second;
    }

    cout << ans << LINE;


}

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

    int t = 1;
    // cin >> t;

    while (t--)
        solve();
}