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// Jan Zakrzewski
#include <bits/stdc++.h>
using namespace std;

int constexpr N = 40;
long long h, w;
int n;
long long d[N];

long long a[N];
long long sum;
long long ans;

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

    cin >> h >> w;
    cin >> n;
    for(int i = 1; i <= n; ++i)
        cin >> d[i];

    for(int i = 1; i <= n; ++i)
        a[i] = (w / d[i] * d[i]) * (h / d[i] * d[i]);

    for(int i = 1; i <= n-1; ++i)
        a[i] -= a[i+1];
    ans = 0;
    sum = 0;
    for(int i = 1; i <= n; ++i) {
        ans += a[i] / (d[i] * d[i]);
        sum += a[i];
    }

    if(sum < h * w) ans = -1;

    cout << ans << "\n";

    return 0;
}