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#include<cstdio>
#include<algorithm>
#include<utility>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<cmath>
#include<iostream>

// macros

#define FORE(c, a, b) for(int c=a; c<= int(b); ++c)
#define FORD(c, a, b) for(int c=a; c>= int(b); --c)
#define FORIT(it, cont, cont_t) for(cont_t::iterator it = cont.begin(); it != cont.end(); ++it)
#define REP(i, n) for(int i = 0; i < (n); ++i)
#define ALL(a) a.begin(), a.end() 
#define PR(n) printf("%d ", (int) (n)) 
#define PRL(n) printf("%lld ", (ll) (n)) 
#define SC(n) scanf("%d", &n);
#define SC2(n, m) scanf("%d%d", &n, &m);

#define xx first
#define yy second
#define pb push_back
#define mp make_pair
#define itr iterator

#define dbg if(1)
#define prd dbg printf

using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned int uint;

typedef vector<int> vi;
typedef set<int> si;
typedef map<int, int> mi;
typedef pair<int, int> pi;
typedef vector<pi> vii;
typedef vector<vi> vvi;

const int NMAX = 1<<20;

int n, h, w;
vi dims;

ll fill(int x, int y) {
  if (min(x,y)==0) return 0;
  int i = n - 1;
  while (i >= 0 && (dims[i] > min(x,y)))
    --i;
  if (i < 0) return -1;

  int dx = x / dims[i], dy = y / dims[i];
  ll core = (ll) dx * dy;
  ll rec1 = fill(x % dims[i], y);
  ll rec2 = fill(dx * dims[i], y % dims[i]);
  if (rec1 < 0 || rec2 < 0) return -1;
  return core + rec1 + rec2;
}

int main() {
  scanf("%d%d%d", &h, &w, &n);
  REP(i, n) {
    int x;
    SC(x);
    dims.pb(x);
  }
  PRL(fill(h,w));
  return 0;
}