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#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <stdio.h>
#include <string.h>
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <cassert>
using namespace std;
 
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 long long
#define all(x) x.begin(), x.end()
#define pb push_back
#define ii pair<int,int>
#define vi vector<int>
#define vii vector<ii>
#define vvi vector<vi>
#define fi first
#define se second
#define ll long long
#define ld long double
vi d;
int n;
map<pair<int,int>, int> mp;
int dp(int h, int w) {
    if(h == 0 || w == 0) return 0;
    if(mp[{h,w}] != 0) return mp[{h,w}];
    int cnt = 0;
    while(cnt < n && d[cnt] <= min(h,w)) cnt++;
    cnt--;
    int cnth = h / d[cnt], cntw = w / d[cnt];
    int ans = min(dp(h - cnth * d[cnt], cntw * d[cnt]) + dp(h, w - cntw * d[cnt]), 
    dp(h - cnth * d[cnt], w) + dp(d[cnt] * cnth, w - cntw * d[cnt])) + cnth * cntw;
    mp[{h,w}] = ans;
    return ans;
}
void test_case() {
    int h, w;
    cin >> h >> w;
    cin >> n;
    d.resize(n);
    for(int i = 0; i < n; i++) cin >> d[i];
    if(h % d[0] != 0 || w % d[0] != 0) {
        cout << "-1\n";
        return;
    }
    cout << dp(h, w) << "\n";
}
int32_t main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
 
    int tc = 1;
    // cin >> tc;
    while(tc--) {
        test_case();
    }
    return 0;
}