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//
// Mateusz Pietrowcow
//
 
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
 
#define MOD 1000000007
#define INF 1000000000
#define INFL 1000000000000000000LL
#define Tak cout << "TAK" << '\n'
#define Nie cout << "NIE" << '\n'
#define min3(x, y, z) min(x, min(y, z))
#define max3(x, y, z) max(x, max(y, z))
 
using namespace std;
 
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int,int> ii;
typedef pair<long long, long long> pll;
typedef pair<unsigned long long, unsigned long long> pull;

ull t[32], r = 0;

void recSolve (ull h, ull w, ull pos)
{
    while (h < t[pos] && w < t[pos])
        pos--;
    r += (h / t[pos]) * (w / t[pos]);

    bool fullH = (h % t[pos] == 0),
         fullW = (w % t[pos] == 0);
    
    if (pos == 0 || (fullH && fullW)) return;
    else if (fullH) recSolve(h, w % t[pos], pos - 1);
    else if (fullW) recSolve(h % t[pos], w, pos - 1);
    else
    {
        recSolve(h - (h % t[pos]), w % t[pos], pos - 1);
        recSolve(h % t[pos], w, pos - 1);
    }
}

void readSolve()
{
    ull n, h, w;
    cin >> h >> w >> n;

    for (int i = 0; i < n; i++)
        cin >> t[i];
    
    if (h % t[0] != 0 || w % t[0] != 0)
    {
        cout << -1;
        return;
    }

    recSolve(h, w, n - 1);
    cout << r;
}
 
int main()
{
    ios_base::sync_with_stdio(0);
    cout.tie(0);
    cin.tie(0);
 
    readSolve();
}