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#include <bits/stdc++.h>
using namespace std;
long long h,w;
long long d[40];
int n;
long long result;

int main(){
    cin>>h>>w;
    cin>>n;
    for(int i=1;i<=n;i++)cin>>d[i];
    
    if(h%d[1] != 0 || w%d[1] != 0){
        cout<<"-1";
        return 0;
    }
    
    for(int i=1;i<=n;i++){
        result += (h/d[i]) * (w/d[i]);
        if(i == 1) continue;
        long long a = (h/d[i]) * d[i];
        long long b = (w/d[i]) * d[i];
        result -= (a/d[i-1]) * (b/d[i-1]);
    }
    
    cout<<result;
}