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#include <iostream>
#include <vector>

using namespace std;

vector<int> pictures;

long long solve(int height, int width, int maxIndex) {
    int max = pictures[maxIndex];
    if (max > height || max > width) {
        if (maxIndex > 0) {
            return solve(height, width, maxIndex - 1);
        }
        return -1;
    }
    long long maxRows = height / max;
    long long maxColumns = width / max;
    int maxWidth = maxColumns * max;
    int maxHeight = maxRows * max;
    if (maxIndex == 0) {
        if (height > maxHeight || width > maxWidth) {
            return -1;
        } else {
            return maxRows * maxColumns;
        }
    }
    long long below = 0;
    if (height > maxHeight) {
        below = solve(height - maxHeight, maxWidth, maxIndex - 1);
        if (below == -1) {
            return -1;
        }
    }
    long long right = 0;
    if (width > maxWidth) {
        right = solve(height, width - maxWidth, maxIndex - 1);
        if (right == -1) {
            return -1;
        }
    }
    return below + right + maxRows * maxColumns;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int height, width;
    cin >> height >> width;

    int n;
    cin >> n;

    for (int i = 0; i < n; i++) {
        int picture;
        cin >> picture;
        pictures.push_back(picture);
    }

    long long num = solve(height, width, pictures.size() - 1);
    cout << num << endl;
}