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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

//#pragma GCC target ("avx2")
//#pragma GCC optimize ("Ofast")
//#pragma GCC optimize ("unroll-loops")

#define f first
#define s second
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((int) (x).size())
#define pb push_back
#define mp make_pair
#define int long long

using namespace std;
using namespace __gnu_pbds;

template <typename T> using oset = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename T> inline bool umin(T &a, const T &b) { if(a > b) { a = b; return 1; } return 0; }
template <typename T> inline bool umax(T &a, const T &b) { if(a < b) { a = b; return 1; } return 0; }

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll mod = 998244353;
const ll base = 1e6 + 9;
const ll inf = 1e18;
const int MAX = 3e5 + 42;
const int LG = 20;

//random_device rd;
//mt19937 gen(rd());
//uniform_int_distribution<ll> dis(1, inf);

void solve() {
    int h, w;
    cin >> h >> w;
    int n;
    cin >> n;
    vector<int> a(n);
    for(auto &i : a) {
        cin >> i;
    }
    int ans = 0;
    reverse(all(a));
    int x = h, y = w;
    for(auto i : a) {
        int can_vert = h / i;
        int can_hor = w / i;
        int cx = x / i;
        int cy = y / i;
        ans += cx * can_hor;
        ans += cy * can_vert;
        ans -= cx * cy;
        x %= i; y %= i;
    }
    cout << (x == 0 && y == 0? ans : -1) << '\n';
}

signed main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int ttt = 1;
//    cin >> ttt;
    while(ttt--) {
        solve();
    }
}