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

#ifndef LOCAL
#pragma GCC optimize("O3")
#endif

#define fi first
#define se second
#define pii pair<int,int>
#define mp make_pair
#define endl '\n'
#define sp <<" "<<
#define eb emplace_back
#define MOD 1000000007
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) (a*(b/gcd(a,b)))
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()

using ll = long long;
#define vec vector

template <class T> void print_v(vector<T> &v) { cout << "{"; for (auto x : v) cout << x << ","; cout << "}\n"; }
template <class T> void print_m(vector<vector<T>> &m) { for (auto v : m) print_v(v); cout << '\n'; }

#define fora(a) for(auto e:a)
#define it(i,s,e) for(long long int i=s;i<e;i++)
#define ita(i,s,e) for(long long int i=s;i<=e;i++)
#define itr(i,e,s) for(long long int i=e-1;i>=s;i--)
#define urs(r...)typename decay<decltype(r)>::type
#define rep(i,n)for(urs(n)i=0;i<(n);++i)


vec<long> sizes;

map<pair<int,int>, long> mem;

long biggest_poss(int n) {
    for (long s: sizes) {
        if (s<=n) return s;
    }

    return -1;
}

long solve(int x, int y) {
    //cout << x sp y << endl;
    if (x == 0 || y == 0) return 0;
    if (x < y) swap(x,y);
    if (mem.count(mp(x,y))) {
        return mem[mp(x,y)];
    }

    long side = min(biggest_poss(x), biggest_poss(y));

    //cout << "side " << side << endl;

    if (side == -1 || side == -1) return -1;

    long rect_x = side * (x/side);
    long rect_y = side * (y/side);

    //cout << "inner " << rect_x sp rect_y << endl;

    long rect1_res = solve(rect_x, y - rect_y);
    long rect2_res = solve(x - rect_x, rect_y);
    long rect3_res = solve(x - rect_x, y - rect_y);

    if (rect1_res == -1 || rect2_res == -1 || rect3_res == -1) return -1;

    return (x/side)*(y/side) + rect1_res + rect2_res + rect3_res;
}

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

    long h,w;
    int n;
    cin >> h >> w >> n;

    rep (_, n) {
        int s;
        cin >> s;
        sizes.eb(s);
    }
    
    sort(sizes.begin(), sizes.end(), greater());

    cout << solve(h, w) << endl;

    return 0;
}