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#include <bits/stdc++.h>
using namespace std;
using ll = long long;

#ifdef DEBUG
#include "debug.h"
#else
#define debug(...) 42
#endif

#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)

#define rep(i, n) for (int i = 0; i < (n); ++i)
#define repp(i, n, m) for (int i = (n); i < (m); ++i)

#define repr(i, n) for (int i = (n) - 1; i >= 0; --i)
#define reppr(i, n, m) for (int i = (m) - 1; i >= (n); --i)

#define each(a,x) for(auto& a : (x))
#define sz(x) int((x).size())

using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using pi = pair<int, int>;
using pll = pair<ll, ll>;

template <typename T, typename V> void mini(T& a, V b) {if (b < a) a = b; }
template <typename T, typename V> void maxi(T& a, V b) {if (b > a) a = b; }

void solve() {
    ll h, w, htaken = 0, wtaken = 0, ans = 0;
    int n;
    cin >> h >> w >> n;

    vll dlist(n);
    rep(i, n)
        cin >> dlist[i];
    sort(rall(dlist));

    for (ll d : dlist) {
        ll newht = h / d * d;
        ll newwt = w / d * d;

        ans += (newht / d) * (newwt / d) - (htaken / d) * (wtaken / d);
        debug(newht, newwt, ans);
        htaken = newht;
        wtaken = newwt;
    }

    if (h != htaken || w != wtaken) {
        cout << "-1\n";
        return;
    }

    cout << ans << '\n';
}

int main() {
#ifdef DEBUG
    const int MEMSIZE = 1024 * 1024 * 1024;
    static_assert(MEMSIZE % 16 == 0);
    static char stack[MEMSIZE];
    asm volatile (
        "mov %[newstack], %%rsp\n"
        "call *%[funcptr]"
        :: [funcptr] "r" (solve), [newstack] "r" (stack + MEMSIZE)
    );
    exit(0);
#else
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    solve();
#endif
}