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//#pragma GCC optimize("Ofast")
//#pragma GCC optimize ("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

#pragma warning(disable:4786)
#pragma warning(disable:4996)
#include <bits/stdc++.h>
#include  <random>
#include <chrono>
#include <unordered_set>
using namespace std;

#define MEM(a, b) memset(a, (b), sizeof(a))
#define CLR(a) memset(a, 0, sizeof(a))
#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define MIN(a, b) ((a) < (b) ? (a) : (b))
#define ABS(X) ( (X) > 0 ? (X) : ( -(X) ) )
#define S(X) ( (X) * (X) )
#define SZ(V) (int )V.size()
#define FORN(i, n) for(int i = 0; i < n; i++)
#define FORAB(i, a, b) for(int i = a; i <= b; i++)
#define ALL(V) V.begin(), V.end()
#define IN(A, B, C)  ((B) <= (A) && (A) <= (C))
#define AIN(A, B, C) assert(IN(A, B, C))

string to_string(string s) { return '"' + s + '"'; }
string to_string(const char* s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <typename A, typename B>
string to_string(pair<A, B> p) { return "(" + to_string(p.first) + ", " + to_string(p.second) + ")"; }

template <typename A>
string to_string(A v) {
    bool first = true;
    string res = "{";
    for (const auto& x : v) {
        if (!first) {
            res += ", ";
        }
        first = false;
        res += to_string(x);
    }
    res += "}";
    return res;
}

void dbg_out() { cerr << endl; }
template<typename Head, typename... Tail>
void dbg_out(Head H, Tail... T) { cerr << ' ' << to_string(H); dbg_out(T...); }
#ifdef LOCAL
#define dbg(...) cerr << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#else
#define dbg(...) 0
#endif

typedef long long int LL;
//typedef __int128 LLL;
typedef long long LLL;


typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef pair<LL, int> PLI;
typedef pair<double, double> PDD;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<PLL> VPL;
typedef vector<PII> VP;
typedef vector<double> VD;
typedef vector<vector<int>> VVI;
typedef vector<vector<LL>> VVL;
typedef vector<string> VS;
typedef long double ld;
typedef unsigned long long ULL;

//#define MAXN 1000
//#define MAXN2 MAXN*MAXN

// mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
// mt19937 rng(10);
// shuffle(V.begin(), V.end(), rng);
// int u = rng();
// int u = uniform_int_distribution<long long>(L, R)(rng);
// double u = uniform_real_distribution<float>(L, R)(rng);

// int dr[] = {-1, 0, 1, 0, 1, 1, -1, -1};
// int dc[] = {0, 1, 0, -1, 1, -1, 1, -1};

const LL MOD = 1000000007;
//const LL MOD = 998244353;
// const LL INF = 2000000000000000001LL; //2e18 + 1

//typedef long long LL;
//typedef unsigned long long ULL;

LL calc(VI& V, int id, int w, int h) {
    LL d = V[id];
    if (w % d == 0) return (w / d) * (h / d);

    int k = h / d;
    LL ret = calc(V, id - 1, w % V[id], d) + (w / d);
    return ret * k;

}

void solve(int ks) {
    int h, w;
    scanf("%d %d", &h, &w);
    int n;
    scanf("%d", &n);
    VI V(n);
    FORN(i, n) scanf("%d", &V[i]);

    if ((h % V[0]) || (w % V[0])) {
        printf("-1\n");
        return;
    }

    LL ans = 0;
    for (int i = n - 1; i >= 0; i--) {
        if (h == 0) break;
        int d = V[i];
        if (h < d) continue;
        LL k = h / d;
        h %= d;
        ans += calc(V, i, w, d * k);
    }
    printf("%lld\n", ans);
}

void gen() {
}

int main() {
    // ios_base::sync_with_stdio(false);
    // cin.tie(nullptr);

    double start_time = clock();
#ifdef LOCAL
    freopen("C:\\Home\\Contest\\sample.in", "r", stdin);
    //freopen("C:\\Home\\Contest\\0.out", "w", stdout);
#endif

    gen();

    if (0) {
        int T;
        scanf("%d", &T);
        for (int ks = 1; ks <= T; ks++) {
            solve(ks);
            if (ks % 100 == 0) fprintf(stderr, "%d done\n", ks);
            //double end_time = clock();
            //fprintf(stderr, "Time = %lf\n", (end_time - start_time) / CLOCKS_PER_SEC);
        }
    }
    else {
        solve(1);
    }

    double end_time = clock();
    fprintf(stderr, "Time = %lf\n", (end_time - start_time) / CLOCKS_PER_SEC);
    return 0;
}