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

long long w,h;
int n;
long long d[60];

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin>>h>>w>>n;
    for(int i=0;i<n;i++) {
        cin >> d[i];
    }
    if(w % d[0] != 0 || h % d[0] != 0) {
        cout << -1;
        return 0;
    }
    long long ret = (w/d[0]) * (h/d[0]);
    for(int i=1;i<n;i++) {
        if(d[i] > w || d[i] > h) {
            break;
        }
        long long cover = (w/d[i]) * (h/d[i]);
        if(cover > 0) {
            long long mult = d[i] / d[i-1];
            ret -= cover * mult * mult;
            ret += cover;
        }
    }
    cout << ret;
    return 0;
}