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

vector<long long> dimension;

long long solve(long long w, long long h, int lvl){
    while(min(w, h) < dimension[lvl]){
        lvl--;
        if(lvl == -1){
            return -1;
        }
    }
    long long w_mod = w/dimension[lvl];
    long long h_mod = h/dimension[lvl];
    long long ans = w_mod * h_mod;
    if(w%dimension[lvl] != 0){
        long long tmp = solve(w%dimension[lvl], dimension[lvl] * h_mod, lvl);
        if(tmp == -1){
            return -1;
        }
        ans += tmp;
    }
    if(h%dimension[lvl] != 0){
        long long tmp = solve(w, h%dimension[lvl], lvl);
        if(tmp == -1){
            return -1;
        }
        ans += tmp;
    }
    return ans;

}


int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    int w, h;
    cin >> w >> h;
    int n;
    cin >> n;
    for(int i = 0; i < n; i++){
        int a;
        cin >> a;
        dimension.push_back(a);
    }
    long long ans = solve(w, h, n - 1);
    cout << ans << '\n';

    return 0;
}