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#include <iostream>

using namespace std;

int d[31];

long long getCount(int h, int w, int index) {
    if(h == 0 || w == 0) {
        return 0;
    }
    if(index < 0) {
        return -1;
    }
    for(int i = index; i >= 0; i--) {
        if(h >= d[i] && w >= d[i]) {
            long long count = (((long long)h) / d[i]) * (((long long)w) / d[i]);
            long long overX = getCount(h % d[i], w - (w % d[i]), index-1);
            long long overY = getCount(h - (h % d[i]), w % d[i], index-1);
            long long overXY = getCount(h % d[i], w % d[i], index-1);

            if(overX < 0 || overY < 0 || overXY < 0) {
                return -1;
            }

            return count + overX + overY + overXY;
        }
    }

    return -1;
}

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

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

    int a;
    for (int i = 0; i < n; i++) {
        cin >> a;
        d[i] = a;
    }

    long long res = getCount(h, w, n-1);

    cout << res << endl;

    return 0;
}