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#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

vector<int> d;

typedef long long LL;

LL ile(int h, int w)
{
    if (h==0 || w==0)
        return 0;
    int x=min(h, w);
    int dd=-1;
    for (int i=d.size()-1; 0<=i; --i)
        if (d[i]<=x)
        {
            dd=d[i];
            break;
        }
    if (dd==-1)
        return -1;

    int hh=h/dd, h3=hh*dd;
    int ww=w/dd, w3=ww*dd;

    LL a=ile(h3, w-w3);
    if (a==-1)
        return -1;
    LL b=ile(h-h3, w3);
    if (b==-1)
        return -1;
    LL c=ile(h-h3, w-w3);
    if (c==-1)
        return -1;

    return ((LL) hh)*ww+a+b+c;
}

int main()
{
    int h, w, n;
    cin>>h>>w>>n;
    for (int i=0; i<n; ++i)
    {
        int a;
        cin>>a;
        d.push_back(a);
    }
    cout<<ile(h, w);
    return 0;
}