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

const int N = 30;
ll tab[N];
ll tools[N];

void makeTools(int z, int n, int w){
    ll ans = 0, left = w;
    for(int i = z ; i >= 0 ; i--){
        ans += (left / tab[i]) * (tab[z] / tab[i]);
        left %= tab[i];
    }
    tools[z] = ans;
}

ll solve(int n, int w, int h){
    ll left = h, ans = 0; 
    for(int i = 0 ; i < n ; i++) makeTools(i, n, w);
    for(int i = n - 1 ; i >= 0 ; i--){
        ans += tools[i] * (left / tab[i]);
        left %= tab[i];
    }
    return ans;
}

int main(){
    int n, w, h;
    scanf("%d %d\n%d", &h, &w, &n);
    for(int i = 0 ; i < n ; i++){
        scanf("%lld", &tab[i]);
    }
    if(w % tab[0] != 0 || h % tab[0] != 0) printf("-1\n");
    else printf("%lld\n", solve(n, w, h));
}