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#include <bits/stdc++.h>
using namespace std;
long long n, m1, m2, t[40];
bool failed;
long long fit(long long a, long long b, long long s)
{
    if (a > b)  swap(a, b);

    if (a == 0)  return 0;

    if (s == -1)
    {
        failed = 1;
        return 0;
    }

    if (a < t[s])  return fit(a, b, s - 1);

    else
    {
        long long x = a / t[s], y = b / t[s];
        x *= y;
        long long n1 = a % t[s], n2 = b % t[s];
        return x + fit(n1, b, s - 1) + fit(n2, a - n1, s - 1);
    }
}
int main()
{
    scanf("%lld%lld%lld", &m1, &m2, &n);
    for (int i = 0; i < n; ++i)
    {
        scanf("%lld", &t[i]);
    }
    long long int x = fit(m1, m2, n-1);
    if (!failed)  printf("%lld", x);

    else  printf("-1");
    return 0;
}