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#include <iostream>
#include <vector>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>

using namespace std;

void dfs(int v, vector<unordered_set<int>> &edges, vector<bool> &vis, 
    vector<int> &topo) {
    if (vis[v]) return;
    vis[v] = true;

    for (auto &u : edges[v]) {
        if (vis[u]) continue;

        if (edges[0].find(u) == edges[0].end()) {
            topo.push_back(u);
        }
        dfs(u, edges, vis, topo);
    }
}

long long dp(vector<int> d, int h, int w) {
    // cout << h << ", " << w << endl;
    if (h == 0 || w == 0) return 0;
    if (d.empty()) return -1;

    int di = d.back();
    
    int a = h / di;
    int b = w / di;

    long long cnt = (long long) a * (long long) b;

    d.pop_back();

    long long ret1 = dp(d, h, w % di);
    long long ret2 = dp(d, h % di, w / di * di);

    if (ret1 == -1 || ret2 == -1) return -1;

    return ret1 + ret2 + cnt;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);


    int h, w, n;
    cin >> h >> w >> n;
    vector<int> d(n);
    for (int i = 0; i < n; ++i) {
        cin >> d[i];
    }
    cout << dp(d, h, w) << "\n";
}