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#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
#define INF 1000000000
#define INFl 1000000000000000000
#define all(x) x.begin(), x.end()
#define sajz(x) (int)x.size()
#define pb push_back
#define se second
#define fi first
#define yes puts("YES")
#define no puts("NO")
#define erase_duplicates(x) {sort(all(x));(x).resize(distance((x).begin(), unique(all(x))));}
using namespace std;
//using namespace __gnu_pbds;
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}

template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif

#define int ll

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef set<int> si;
typedef multiset<int> msi;
typedef long double ld;
//typedef cc_hash_table<int, int, hash<int>> ht;

int d[31];
int n;

bool bad = false;

int solve(int h, int w) {
    if (h == 0 || w == 0) return 0;

    if (min(h, w) < d[0]) {
        bad = true;
        return -1;
    }

    int best = -1;

    if (h > w) swap(h, w);

    for (int i = n - 1; i >= 0; i --) {
        if (h >= d[i]) {
            best = i;
            break;
        }
    }

    int ile_klade = (w / d[best]) * (h / d[best]);

    //debug(h, w, ile_klade, d[best]);

    return ile_klade + solve(h - h / d[best] * d[best], w / d[best] * d[best]) + solve(h, w - w / d[best] * d[best]);;
}

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

    int h, w;
    cin >> h >> w;
    cin >> n;
    for (int i = 0; i < n; i ++) cin >> d[i];

    int res = solve(h, w);
    if (bad) cout << -1 << '\n';
    else cout << res << '\n';

    return 0;
}