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/* -----------------------
Autor: Tomasz Boguslawski
-------------------------- */
#include<cstdio>
#include<cstdlib>
#include<iostream>
#include<fstream>
#include<iomanip>
#include<string>
#include<sstream>
#include<cstring>
#include<map>
#include<vector>
#include<set>
#include<queue>
#include<algorithm>
#include <fstream>
#include<math.h>

#define LL long long
#define FOR(x, b, e) for(LL x = b; x <= (e); x++)
#define FORS(x, b, e, s) for(LL x = b; x <= (e); x+=s)
#define FORD(x, b, e) for(LL x = b; x >= (e); x--)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define DEBUG if (debug)
#define MIN(a,b) ((a>b)?b:a)
#define MAX(a,b) ((a>b)?a:b)

using namespace std;

LL total;
LL n;
LL sizes[100];

class Rectangle
{
public:
    LL w, h;
    Rectangle(LL p_w, LL p_h) { w=p_w; h=p_h; }
};

vector<Rectangle> rectangles;

bool handleRectangle()
{
    Rectangle rec = rectangles.back(); rectangles.pop_back();
    //cout << "Handling " << rec.w << " x " << rec.h << "\n";
    LL pas=0;
    FOR(i,1,n) if ((sizes[i]<=rec.w)&&(sizes[i]<=rec.h)) { pas=sizes[i]; break; }
    if (pas==0) return false;
    //cout << "Chosen: " << pas << "\n";
    total+=(rec.w/pas)*(rec.h/pas);
    LL w2=rec.w%pas;
    if (w2!=0) rectangles.push_back(Rectangle(w2, rec.h));
    LL h2=rec.h%pas;
    if (h2!=0) rectangles.push_back(Rectangle(pas*(rec.w/pas), h2));
    return true;
}



/// MAIN
int main(int argc, char* argv[])
{
    // magic formula, which makes streams work faster:
	ios_base::sync_with_stdio(0);
	total=0;
	LL w,h; cin >> h; cin >> w;
	cin >> n;
	FORD(i,n,1) cin >> sizes[i];
	rectangles.push_back(Rectangle(w,h));
	bool ok=true;
	while (rectangles.size()!=0)
    {
        ok = handleRectangle();
        if (!ok) { cout << "-1\n"; return 0; }
    }
    cout << total << '\n';
	return 0;
}