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#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define int long long
#define st first
#define nd second
#define pb push_back
#define mp make_pair
#define rep(i,a,b) for(ll i=(ll)a;i<=(ll)b;++i)
#define all(a) a.begin(),a.end()
#define sz(x) (int)(x).size()
const int mxN = 37;
int n, a[mxN];

int policz(int w,int h){
    if(w == 0) return 0;
    int d = 0;
    rep(i,1,n) if(a[i] <= w) d = a[i];

    int res = (w/d) * (h/d);
    int rw = w%d;
    int rh = h%d;

    res += policz(min(rw, h-rh), max(rw, h-rh));
    res += policz(min(rh, w), max(rh, w));
    return res;
}

int32_t main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int h,w; cin >> h >> w;
    cin >> n;
    rep(i,1,n) cin >> a[i];
    sort(a+1,a+n+1);

    if(w%a[1] != 0 || h%a[1] != 0) cout << "-1\n";
    else cout << policz(min(w,h), max(w,h)) << "\n";
}