import java.util.*;
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
long x = in.nextInt();
long y = in.nextInt();
int n = in.nextInt();
int[] timesArray = new int[n];
int i = 0;
while (i < n) { // 注意 while 处理多个 case
timesArray[i] = in.nextInt();
i++;
// int b = in.nextInt();
// System.out.println(a + b);
}
if (x >= y) {
System.out.println(0);
return;
}
Arrays.sort(timesArray);
int times = 0;
int lastNum = -1;
for (int j = timesArray.length - 1; j >= 0; j--) {
int CurNum = timesArray[j];
if (CurNum == lastNum) {
continue;
}
x *= CurNum;
times++;
lastNum = CurNum;
if (x >= y) {
break;
}
}
if (x < y) {
times = -1;
}
System.out.println(times);
// 注意 hasNext 和 hasNextLine 的区别
// while (in.hasNextInt()) { // 注意 while 处理多个 case
// int a = in.nextInt();
// int b = in.nextInt();
// System.out.println(a + b);
// }
}
}