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#include <bits/stdc++.h>

using namespace std;

typedef int64_t i64;

void solve() {
    i64 h, w, n;
    cin >> h >> w >> n;
    vector<i64> d(n);
    for (i64 i = 0; i < n; i++) {
        cin >> d[i];
    }

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

    i64 ans = 0;

    i64 i = n - 1;
    i64 tmpH = h;
    while (tmpH > 0) {
        while (d[i] > tmpH || d[i] > w) {
            i--;
        }
        i64 tmpAns = 0;
        i64 j = i;
        i64 tmpW = w;
        i64 mult = 1;
        while (tmpW > 0) {
            tmpAns += (tmpW / d[j]) * mult;
            tmpW %= d[j];
            j--;
            if (j >= 0) {
                mult *= d[j + 1] / d[j];
            }
        }
        ans += (tmpH / d[i]) * tmpAns;
        tmpH %= d[i];
    }

    cout << ans << "\n";
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    
    solve();
  
    return 0;
}