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#include <bits/stdc++.h>

using namespace std;

#define ll long long

#define rng(i,a,b) for(int i=int(a);i<int(b);i++)
#define rep(i,b) rng(i,0,b)

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;

typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;

typedef pair<int,int> ii;

template<class t> using vc=vector<t>;
template<class t> using vvc=vc<vc<t>>;

const int MOD = 998244353;

ll read(){
    ll i;
    cin>>i;
    return i;
}

vi readvi(int n,int off=0,int shift=0){
    vi v(n+shift);
    rep(i,shift)v[i]=0;
    rep(i,n)v[i+shift]=read()+off;
    return v;
}

void YesNo(bool condition, bool do_exit=true) {
    if (condition)
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
    if (do_exit)
        exit(0);
}

ll solve(ll h, ll w, int n, vi & d) {
    if (w < h)
        swap(h,w);
    // w >= h

    if (h == 0)
        return 0;

    ll cnt = 0;
    ll hh = h;
    ll fit = -1;
    for (int i = n-1; hh > 0 and i>=0; --i) {
        ll inc = hh / d[i];
        if (fit == -1 and inc > 0)
            fit = d[i];
        hh = h % d[i];
        cnt += inc * (fit / d[i]);
    }

    cnt *= (w / fit);

    return cnt + solve(h, w % fit, n, d);


}

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

    ll h,w;
    cin >> h >> w;
    int n;
    cin >> n;
    vi d = readvi(n);

    if (h % d[0] != 0 or w % d[0] != 0) {
        cout << -1 << "\n";
        return 0;
    }

    cout << solve(h, w, n, d);

    return 0;
}