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#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<cmath>
#include<queue>
#include<queue>
#include<stack>
#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef vector<LL> VLL;
typedef vector<LD> VLD;
typedef vector<VLL > VVLL;
typedef vector<VLD > VVLD;
typedef vector<PII > VPII;
typedef __int128 int128;
template<class TIn>
using indexed_set = tree<
        TIn, null_type, less<TIn>,
        rb_tree_tag, tree_order_statistics_node_update>;
template<class T> using min_heap = priority_queue<T, vector<T>, greater<T>>;

#define FOR(x, b, e) for(int x=b; x<=(e); ++x)
#define FORD(x, b, e) for(int x=b; x>=(e); --x)
#define REP(x, n) for(int x=0; x<(n); ++x)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define PB push_back
#define ST first
#define ND second
#define THIS (*this)
#define LSB(x) (x & -x)
#define SQR(x) ((x)*(x))

int main() {
	ios_base::sync_with_stdio(0);
    cin.tie(NULL); cout.tie(NULL);
    int h, w; cin >> h >> w;
    int n; cin >> n;
    VI d(n);
    REP(i, n) cin >> d[i];
    LL ans = 0, hLeft = h, wLeft = w;
    FORD(i, n - 1, 0) {
        LL hCnt = hLeft / d[i];
        LL wCnt = wLeft / d[i];
        ans += (h - hLeft) / d[i] * wCnt;
        ans += (w - wLeft) / d[i] * hCnt;
        ans += hCnt * wCnt;
        hLeft -= hCnt * d[i];
        wLeft -= wCnt * d[i];
    }
    if (hLeft || wLeft) cout << "-1\n";
    else cout << ans << '\n';

    return 0;
}