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

#define int int64_t
#define pb push_back
#define pii pair<int, int>
#define st first
#define nd second
#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()

const int INF = 1'000'000'000'000'000'010;

vector<int> v;

int cnt = 0;
int f(int n, int m, int I) {
	cnt++;
	// cerr << "f: " << n << " " << m << " " << I << "\n";
	if(n==0||m==0) return 0;
	if(I==sz(v)) {
		return INF;
	}
	int val = v[I];
	if(val>n||val>m) return f(n, m, I+1);
	int cur = 0;
	cur += (n/val)*(m/val);
	cur += f(n, m%val, I+1);
	cur += f(n%val, m/val*val, I+1);
	return min(cur, INF);
}

int32_t main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	
	int n, m;
	cin >> n >> m;
	int x;
	cin >> x;
	v.resize(x);
	for(int i=0;i<x;i++) {
		cin >> v[i];
	}
	sort(all(v));
	reverse(all(v));

	int ans = f(n, m, 0);
	if(ans==INF) {
		cout << "-1\n";
	} else {
		cout << ans << "\n";
	}
	// cout << cnt << "\n";
}