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//Sylwia Sapkowska
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
using namespace std;

void __print(int x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << "'" << x << "'";}
void __print(const char *x) {cerr << '"' << x << '"';}
void __print(const string &x) {cerr << '"' << x << '"';}
void __print(bool x) {cerr << (x ? "true" : "false");}

template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif

#define int long long
typedef pair<int, int> T;
const int oo = 1e18, oo2 = 1e9+7, K = 30;
const int mod = 998244353;

void solve(){
    int n, m; cin >> n >> m;
    int k; cin >> k;
    vector<int>a(k);
    for (auto &x: a) cin >> x;
    if (n%a[0] != 0 || m%a[0] != 0) {
        cout << "-1\n";
        return;
    }
    n/=a[0];m/=a[0];
    for (int i = (int)a.size()-1; i>=0; i--){
        a[i]/=a[0];
    }
    debug(n, m, a);
    int ans = 0;
    function<void(int, int)>rec = [&](int N, int M){
        if (N <= 1) {
            ans += N*M;
            return;
        }
        int t = M;
        int d = 1;
        for (int i = 1; i<k; i++) if (a[i] <= N) d = a[i];
        //d x M
        int curr = N-N%d; 
        debug(N, M, d, curr);
        for (int i = k-1; i>=0; i--){
            int v = (t/a[i])*(curr/a[i]); 
            debug(a[i], v);
            ans += v;
            if (v > 0) t %= a[i];
        }
        if (curr != N) rec(N%d, M); //pasek na gorze
    };
    if (n > m) swap(n, m);
    rec(n, m);
    cout << ans << "\n";
}

int32_t main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    int t = 1;
    //cin >> t;
    while (t--) solve();

    return 0;
}