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#include <bits/stdc++.h>

using namespace std;

#define int long long


void solve(){
	
	int h, w, n;
	cin >> h >> w >> n;
	
	vector<int> v(n);
	for(auto &u : v) cin >> u;
	
	reverse(v.begin(), v.end());
	int ans = 0;
	
	for(int i = 0; i < n; i++){
		
		if(v[i] <= h && v[i] <= w){
			
			int gora = h;
			int bok = w - w%v[i];
			w %= v[i];
			
			for(int j = i; j < n; j++){
				
				if(v[j] <= gora){
					
					int ilebok = bok/v[j];
					int ilegora = gora/v[j];
					
					ans += ilebok * ilegora;
					gora -= ilegora * v[j];
				}		
			}
			
			if(gora != 0){
				
				cout << -1 << '\n';
				return;
			}
		}
	}
	
	if(w != 0) cout << -1 << '\n';
	else cout << ans << '\n';
}


signed main(){
	
	ios_base::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);
	
	int t = 1;
	//cin >> t;
	
	while(t--) solve();	
	
	return 0;
}