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#include <bits/stdc++.h>
using namespace std;
 
#define fwd(i, a, n) for (int i = (a); i < (n); i ++)
#define rep(i, n) fwd(i, 0, n)
#define all(X) begin(X), end(X)
#define sz(X) ((int)X.size())
#define st first
#define nd second
#define pii pair<int, int>
#define vi vector<int>
#define ll long long
 
#ifdef LOC
auto &operator<<(auto &out, pair<auto, auto> a) {
	return out << "(" << a.st << ", " << a.nd << ")";
}
 
auto &operator<<(auto &out, auto a) {
	out << "{";
	for (auto b : a)
		out << b << ", ";
	return out << "}";
}
 
void dump(auto... x) { ((cerr << x << ", "), ...) << '\n'; }
#define debug(x...) cerr << __LINE__ << ": [" #x "]: ", dump(x)
#else
#define debug(...) 0
#endif
 
int32_t main() {
	ios_base::sync_with_stdio(0), cin.tie(0);
    ll h, w;
    cin>>h>>w;
    int n; cin>>n;
    vector<ll> v(n);
    rep(i, n)cin>>v[i];
    for(int i=  n-1; i >= 1; i--)v[i] /= v[i-1];
    if(h % v[0] == 0 && w % v[0] == 0){
        long long res = 0;
        h /= v[0];
        w /= v[0];
        for(int i = 1; i < n; i++){
            ll a = h % v[i];
            ll b = w % v[i];
            debug(h, w, a, b);
            res += a * w + h * b - a * b;
            h /= v[i];
            w /= v[i];
        }
        debug(h, w);
        res += h * w;
        cout<<res<<"\n";
    }else{
        cout<<"-1\n";
    }
	return 0;
}