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#include <bits/stdc++.h>
using namespace std;

#define mp make_pair
#define pb push_back
// #define st first
#define nd second
#define fi first
#define se second
#define vt vector
#define FOR(a, b, c)  for(int a=b; a<c; ++a)
#define all(a)     (a).begin(),(a).end()
#define sz(a)      (int)(a).size()

typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
typedef vector<int> vi; 
typedef vector<ll> vll;
typedef pii PII;
typedef pll PLL;

constexpr ll nax = 2e5+6969, INF = 1e9+2137;
constexpr ld eps = 1e-9;

mt19937_64 rng(6969);
// mt19937_64 rng(chrono::system_clock::now().time_since_epoch().count());

inline ll rand(ll l, ll r) {
	return uniform_int_distribution<ll>(l, r)(rng);
}

ll w, h;
int n;  
ll ans; 
vll d; 

int32_t main() {
    // ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    cin >> h >> w >> n; 
    if(h > w) 
        swap(h, w); 
    d.resize(n); 

    for(auto &e: d) {
        cin >> e; 
    }

    ll g = __gcd(h, w); 
    if(g % d[0] != 0) {
        puts("-1"); 
        return 0; 
    }
    priority_queue<pll> q; 
    q.push(mp(h, w)); 

    for(int i = n - 1; i >= 0; i--) {
        ll cur = d[i]; 
        while(!q.empty()) {
            pll sz = q.top(); 
            // sz.fi *= -1; 
            // sz.se *= -1; 

            if(sz.fi < cur) {
                break;
            }
            q.pop(); 

            ans += (sz.fi / cur) * (sz.se / cur); 
            ll a = sz.fi % cur; 
            ll b = sz.se / cur * cur; 
            if(a > b) 
                swap(a, b); 
            if(a > 0) 
                q.push(mp(a, b)); 

            a = sz.se % cur; 
            b = sz.fi; 
            if(a > b) 
                swap(a, b); 
            if(a > 0) 
                q.push(mp(a, b)); 
        }
    }

    cout << ans << "\n"; 

    return 0;
}