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#include <cstdio>
using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define INT(x) int x; scanf("%d", &x)

typedef long long LL;

int d[30];

int main() {
	INT(h);
	INT(w);
	INT(n);
	REP(i,n) scanf("%d", &d[i]);
	if (h % d[0] || w % d[0]) {
		printf("-1\n");
		return 0;
	}
	h /= d[0];
	w /= d[0];
	LL r = LL(h) * w;
	REP(i,n-1) {
		LL p = d[i + 1] / d[i];
		h /= p;
		w /= p;
		r -= LL(h) * w * (p * p - 1);
	}
	printf("%lld\n", r);
}