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

typedef long long int LL;

int n;
LL d[40];

LL solve(LL h, LL w, int sIdx)
{
    if(h == 0 || w == 0)
    {
        return 0;
    }

    while(d[sIdx] > h || d[sIdx] > w)
    {
        ++sIdx;
    }

    LL a = d[sIdx++];
    LL wa = w/a;
    LL ha = h/a;
    LL res = wa * ha;

    LL subResLeft = solve(h, w%a, sIdx);
    LL subResRight = solve(h%a, a*wa, sIdx);
    return res + subResLeft + subResRight;
}


int main()
{
    ios::sync_with_stdio(0);

    LL h, w;
    cin >> h >> w;
    cin >> n;
    for(int i = n-1; i >= 0; --i)
    {
        cin >> d[i];
    }

    if ((h % d[n-1] != 0) || (w % d[n-1] != 0))
    {
        cout << -1 << endl;
        return 0;
    }

    cout << solve(h, w, 0) << endl;

    return 0;

}