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#pragma GCC optimize ("Ofast")
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define FOR(i, a, b) for (auto i=(a); i<(b); i++)
#define FORD(i, a, b) for (int i=(a); i>(b); i--)
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define PPC(x) __builtin_popcountll(x)
#define MSB(x) (63 - __builtin_clzll(x))
#define LSB(x) __builtin_ctzll(x)
#define ARG(x, i) (get<i>(x))
#define ithBit(m, i) ((m) >> (i) & 1)
#define pb push_back
#define ft first
#define sd second
#define kw(a) ((a) * (a))
#define CLR(x) x.clear(), x.shrink_to_fit()
#ifdef DEBUG
#include "debug.h"
#else
#define dbg(...) 0
#endif
using namespace std;
 
template <typename T1, typename T2> inline void remin(T1& a, T2 b) { a = min(a, (T1)b);	}
template <typename T1, typename T2> inline void remax(T1& a, T2 b) { a = max(a, (T1)b);	}

const int maxN = 30'003;

vector <int> D;
long long res;

void go(int h, int w, int i)
{
	if (min(h, w) == 0)
		return;
	while (D[i] > min(h, w))
		i--;

	int d = D[i--];
	res += (h / d) * (w / d);

	go(h % d, w - w % d, i);
	go(h - h % d, w % d, i);
	go(h % d, w % d, i);
}

void solve()
{
	int h, w, n;
	scanf ("%d%d%d", &h, &w, &n);
	D.resize(n);
	for (int& d : D)
		scanf ("%d", &d);

	if (h % D.front() + w % D.front())
	{
		printf("-1\n");
		return;
	}

	go(h, w, n-1);
	printf("%lld\n", res);
}

int main()
{
	int t = 1;
//	scanf ("%d", &t);	
	FOR(tid, 1, t+1)
	{
		//printf("Case #%d: ", tid);
		solve();
	}
	return 0;
}