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// Author   : Jakub Rożek
// Task     : Obrazy
// Contest  : PA 2024 r3 C
// Memory   : O(?)
// Time     : O(?)
// Solution : Działa

#include "bits/stdc++.h"
using namespace std;
using LL = long long;
template <typename T>
using P = pair<T, T>;
template <typename T>
using VV = vector<vector<T>>;
#define all(x) x.begin(), x.end()
#define FOR(i,a,b) for(int i=(a); i<=(b); ++i)
#define FORD(i,a,b) for(int i=(a); i>=(b); --i)
#define REP(i,n) for(int i=0; i<(n); ++i)
#define ssize(x) int((x).size())
#ifdef DEBUG
template <typename T1, typename T2>
auto&operator<<(auto&o,pair<T1,T2>p){return o<<'('<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";for(auto e:x)o<<","<<e;return o<<"}";}
#define debug(x...) cerr<<"["#x"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(x)
#else
#define debug(...) {}
#endif

const int INF = 1'000'000'009;
// const int N = 1'000'000;

LL h, w;
int n;
vector <LL> v;

LL licz(LL a, LL b) {
    if (a > b) swap(a, b);
    if (a == 0) return 0;
    LL c = INF;
    for (auto i:v){
        if (i <= a) {
            c = i;
            break;
        }
    }

    LL x = a / c;
    LL y = b / c;
    LL wyn = x * y;

    wyn += licz(a-x*c, y*c);
    wyn += licz(a, b-y*c);

    return wyn;
}

void solution() {
    cin >> h >> w;
    cin >> n;
    v.resize(n);
    FORD (i, n-1, 0) {
        cin >> v[i];
    }
    if (h % v[n-1] || w % v[n-1]) cout << "-1\n";
    else cout << licz(h, w) << '\n';
}

int main() {
    cin.tie(0)->sync_with_stdio(0);
    int tests = 1;
    // cin>>tests;
    FOR (i, 1, tests) {
        solution();
    }
    return 0;
}