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

const int L = 30;
int dims[L] = { 0 };


void printArray(int array[]) {
	for (int i = 0; i < L; i++) {
		cout << array[i] << " ";
	}
	cout << endl;
}

struct Counter {
	long long int h;
	long long int w;
	int maxDimIndex;
	long long int totalCount = 0;


	Counter(long long int initH, long long int initW, int n) {
		this->h = initH;
		this->w = initW;
		this->maxDimIndex = n;
		while (h > 0 && w > 0) {
			process();
		}
	}
	void process() {
		long long int maxIncludedLength = 0;
		for (int i = maxDimIndex; i >= 0; i--) {
			const long long int cDim = dims[i];
			if (h >= cDim && w >= cDim) {
				maxIncludedLength = cDim;
				maxDimIndex = i - 1;
				break;
			}
		}

		long long int mulH = h / maxIncludedLength;
		h = h % maxIncludedLength;

		long long int mulW = w / maxIncludedLength;
		w = w % maxIncludedLength;

		totalCount += mulH * mulW;
		const int rectH = mulH * maxIncludedLength;
		const int rectW = mulW * maxIncludedLength;

		int restH = h, restW = w;
		for (int i = maxDimIndex; i >= 0; i--) {
			const int cDim = dims[i];

			mulW = restW / cDim;
			totalCount += rectH / cDim * mulW;
			restW = restW % cDim;

			mulH = restH / cDim;
			totalCount += rectW / cDim * mulH;
			restH = restH % cDim;
		}

	}

};

int main() {
//	ifstream cin("tests/0a.in");
//	ifstream cin("tests/4.in");

	cin.tie(NULL);
	cout.tie(NULL);
	ios_base::sync_with_stdio(false);

	int n;
	long long int h, w;
	cin >> h;
	cin >> w;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> dims[i];
	}
//	printArray(dims);

	if (h % dims[0] != 0 || w % dims[0] != 0) {
		cout << "-1" << endl;
		return 0;
	}

	Counter ctr(h, w, n - 1);

//	printArray(height);
//	printArray(width);

	cout << ctr.totalCount << endl; // prints
	return 0;
}