#include <iostream>
#include <vector>
int main()
{
    long long h, w;
    std::cin >> h >> w;
    long long n;
    std::cin >> n;
    std::vector<long long> a(n);
    for (int i = 0; i < n; i++)
    {
        std::cin >> a[i];
    }
    if (h % a[0] != 0 || w % a[0] != 0){
        std::cout<<"-1";
        return 0;
    }
    long long currenth = 0, currentw = 0, ans = 0;
    long long cntv, cnth;
    for (int i = n-1; i >=0; i--){
        cnth = (h - currenth)/a[i];
        cntv = (w - currentw)/a[i];
        ans += cntv *currenth/a[i] + cnth * currentw/a[i] + cntv*cnth;
        currenth += cnth * a[i];
        currentw += cntv * a[i];
    }
    std::cout<<ans;
}