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// Arti1990, II UWr

#include <bits/stdc++.h>

#define forr(i, n)                  for(int i=0; i<n; i++)
#define FOREACH(iter, coll)         for(auto iter = coll.begin(); iter != coll.end(); ++iter)
#define FOREACHR(iter, coll)        for(auto iter = coll.rbegin(); iter != coll.rend(); ++iter)
#define lbound(P,K,FUN)             ({auto SSS=P, PPP = P-1, KKK=(K)+1; while(PPP+1!=KKK) {SSS = (PPP+(KKK-PPP)/2); if(FUN(SSS)) KKK = SSS; else PPP = SSS;} PPP;})
#define testy()                     int _tests; cin>>_tests; FOR(_test, 1, _tests)
#define CLEAR(tab)                  memset(tab, 0, sizeof(tab))
#define CONTAIN(el, coll)           (coll.find(el) != coll.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 MP                          make_pair
#define PB                          push_back
#define ff                          first
#define ss                          second
#define deb(X)                      X;
#define SIZE(coll) 					((int)coll.size())

#define M 1000000007
#define INF 1000000007LL

using namespace std;

long long n, w, h, tab[107], res;

int solve()
{ 
    cin >> w >> h >> n;
    forr(i, n)
        cin >> tab[i];
    
    if(w%tab[0] != 0 || h%tab[0] != 0)
    {
        cout << "-1" << '\n';
        return 0;
    }

    long long prev_w = 0, prev_h = 0;
    FORD(i, n-1, 0)
    {
        long long w2 = w/tab[i]*tab[i];
        long long h2 = h/tab[i]*tab[i];
        res += w2/tab[i]*h2/tab[i] - prev_w/tab[i]*prev_h/tab[i];
        //cout << res << '\n';
        prev_w = w2;
        prev_h = h2;
    }
    cout << res << '\n';

    return 0;
}

int main()
{
    std::ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    solve();

    return 0;
}