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

#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define RFOR(i, a, b) for(int i = (a) - 1; i >= (b); i--)
#define SZ(a) int(a.size())
#define ALL(a) a.begin(), a.end()
#define PB push_back
#define MP make_pair
#define F first
#define S second

typedef long long LL;
typedef vector<int> VI;
typedef pair<int, int> PII;
typedef long double db;

int main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
    LL a, b, n;
    cin >> a >> b >> n;
    VI d(n);
    for (int& di : d)
        cin >> di;
    reverse(ALL(d));
    LL ans = 0;
    for (int di : d)
    {
        if (a % di == 0)
        {
            ans += (a / di) * (b / di);
            b %= di;
        }
        else
        {
            ans += (a / di) * (b / di);
            a %= di;
        }
    }
    cout << (a > 0 && b > 0 ? -1 : ans) << "\n";
	return 0;
}