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#include <vector>
#include <cstdio>
#include <stack>
#include <algorithm>
#include <queue>
#include <map>
#include <cmath>
#include <unordered_set>
#include <unordered_map>
#include <set>
#include <cstring>
#include <bitset>
#include <iostream>
#include <queue>
#include <iomanip>
#include <complex>

using namespace std;

#define ALL(x) x.begin(), x.end()
#define UNIQUE(c) (c).resize(unique(ALL(c)) - (c).begin())
#define FOR(i, a, b) for(int i =(a); i <=(b); ++i)
#define RE(i, n) FOR(i, 1, n)
#define RED(i, n) FORD(i, n, 1)
#define FORD(i, a, b) for(int i = (a); i >= (b); --i)
#define REP(i, n) for(int i = 0;i <(n); ++i)
#define REPD(i, n) FORD(i, n-1,0)


const int maxn = 32;
const long long mod = 998244353, inf = 1000000004;

long long d[maxn], na, wa,ha;

long long sol(long long w, long long h, long long n){
    if(w < h)swap(w, h);
    long long res =0;
    if(!h)return res;
    long long wys = (h / d[n - 1]) * d[n - 1];
    if(wys) {
        long long sze = w;
        REPD(i, n){
            long long pom = sze / d[i];
            res += pom * (wys / d[i]);
            sze -= pom * d[i];
        }
    }
    return res + sol(w, h - wys, n - 1);
}

void solve() {
    cin>>wa>>ha>>na;
    REP(i, na){
        cin>>d[i];
    }
    if(wa % d[0] != 0 || ha % d[0] != 0) {
        cout<<"-1\n";
    } else {
        cout << sol(wa, ha, na) << "\n";
    }
}

int main() {
    ios_base::sync_with_stdio(false), cin.tie(nullptr);
    int tt = 1;
//    cin >> tt;
    while (tt--) {
        solve();
    }
    return 0;
}