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//Krzysztof Pieprzak
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef unsigned int uint;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int,int> pii;
typedef pair<long long, long long> pll;
typedef vector<pii> vpii;
typedef vector<pll> vpll;

#define Size(x) (int)x.size()
#define VAR(v,n) auto v = (n)
#define FOR(i,a,b) for(VAR(i,a); i < (b); ++i)
#define FORE(i,a,b) for(VAR(i,a); i <= (b); ++i)
#define FORREV(i,a,b) for(VAR(i,b); i >= (a); --i)
#define FORSTEP(i,a,b,step) for(VAR(i,a); i < (b); i += (step))
#define FOREACH(i,c) for(auto i : (c))
#define FOREACHS(i,c,n) for(VAR(i,&(c)[0]); i-(c)<(n); ++i)
#define ALL(x) x.begin(),x.end()
#define CLEAR(t) memset(t, 0, sizeof(t))
#define F first
#define S second
#define MP make_pair
#define PUB push_back
#define POB pop_back
#define pieprzu ios_base::sync_with_stdio(0);

const int    INF = 1000000001;
const double EPS = 10e-9;

const int MAXN = 30;

int t[MAXN + 5];

void rob(int test) {
    int h, w;
    scanf("%d %d", &h, &w);

    int n;
    scanf("%d", &n);
    FOR (i, 0, n) {
        scanf("%d", &t[i]);
    }

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

    ll leftH = h;
    ll leftW = w;
    ll downH = h;
    ll downW = 0;
    ll result = 0;
    FORREV (i, 0, n - 1) {
        ll addedToLeftH = leftH / t[i];
        ll addedToLeftW = leftW / t[i];
        ll addedToDownH = downH / t[i];
        ll addedToDownW = downW / t[i];

        result += addedToLeftH * addedToLeftW;
        result += addedToDownH * addedToDownW;

        leftW -= addedToLeftW * t[i];
        downH -= addedToDownH * t[i];
        downW += addedToLeftW * t[i];
    }

    printf("%lld\n", result);
}

int main() {
    int test = 1;
    //scanf("%d", &test);

    FORE (i, 1, test) rob(i);

    return 0;
}