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#include <ios>
#include <functional>
#include <cassert>
#include <vector>
#define REP(i, n) for(int i=0; i<(n); ++i)
#define SREP(n) REP(_ ## n, n)
#define FOR(i, p, n) for(int i=(p); i<=(n); ++i)
#define RFOR(i, p, n) for(int i=(p); i>=(n); --i)
#define pb push_back
#define eb emplace_back
#define C const
#define V std::vector
#define F std::function
#define R std::ranges
#define RV std::ranges::views
#define sz(A) int(A.size())
#define all(A) A.begin(), A.end()
#define rall(A) A.rbegin(), A.rend()
typedef long long ll;
typedef long double ld;
typedef V <int> vi;
typedef V <vi> vvi;
typedef V <bool> vb;
typedef V <char> vc;
typedef V <ll> vll;
typedef const int ci;
typedef const ll cll;
int I(){
    int z;
    while ((z=getchar_unlocked())<'0'||z>'9');
    int r=z-'0';
    while ((z=getchar_unlocked())>='0'&&z<='9')
        r=r*10+z-'0';
    return r;
}
int main(){
    int x=I(),y=I();
    ci n=I();
    vi dziel(n);
    REP(i, n)
        dziel[i]=I();
    if (x%dziel[0]||y%dziel[0]){
        printf("-1\n");
        exit(0);
    }
    ll w=0;
    while (x&&y){
        ci l=dziel.back();
        if (l>x||l>y){
            dziel.pop_back();
            continue;
        }
        ci nx=x%l,ny=y%l;
        w+=ll(x/l)*(y/l);
        auto pros=[&](int a, int b) -> void{
            assert(!(a%l));
            RFOR(i, sz(dziel)-1, 0){
                ci j=dziel[i];
                w+=ll(a/j)*(b/j);
                b%=j;
            }
            assert(!b);
        };
        pros(x-nx, ny),pros(y-ny, nx);
        x=nx,y=ny;
    }
    printf("%lld\n", w);
}