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#include <cstdio>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <queue>
#include <list>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <stack>
#include <string>
#include <cstring>
#include <cassert>
#include <limits.h>

using namespace std;

typedef long long LL;
typedef long double LD;

typedef vector<int> VI;
typedef pair<int,int> PII;
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(VAR(i,a);i<=(b);++i)
#define FORD(i,a,b) for(VAR(i,a);i>=(b);--i)
#define FORE(a,b) for(VAR(a,(b).begin());a!=(b).end();++a)
#define VAR(a,b) __typeof(b) a=(b)
#define SIZE(a) ((int)((a).size()))
#define ALL(x) (x).begin(),(x).end()
#define CLR(x,a) memset(x,a,sizeof(x))
#define ZERO(x) memset(x,0,sizeof(x))
#define S(t,i) scanf("%" ## t, &i)
#define SI(i) scanf("%d", &i)

const int INF=INT_MAX;
const int MAXN=30+1;

int h, w;
int n;
int d[MAXN];

LL solve(LL w, LL h, int n) {
    while(d[n] > w || d[n] > h) {
        n--;
    }
    LL result = 0;
    result += (w/d[n]) * (h/d[n]);
    if (w%d[n]) {
        result += solve(w%d[n], d[n] * (h/d[n]), n);
    }
    if (h%d[n]) {
        result += solve(d[n] * (w/d[n]), h%d[n], n);
    }
    if (w%d[n] > 0 && h%d[n] > 0) {
        result += solve(w%d[n], h%d[n], n);
    }
    return result;
}

int main() {
    ios_base::sync_with_stdio(0);

    SI(h);
    SI(w);
    SI(n);
    FOR(i, 1, n) {
        SI(d[i]);
    }
    if (w % d[1] > 0 || h % d[1] > 0) {
        printf("-1\n");
        return 0;
    }
    printf("%lld\n", solve(w, h, n));

    return 0;
}