# Will two kangaroos ever meet after making same number of jumps?

There are two kangaroos on an x-axis ready to jump in the positive direction (i.e, toward positive infinity). The first kangaroo starts at location $$x_1$$ and moves at a rate of $$v_1$$ meters per jump. The second kangaroo starts at location $$x_2$$ and moves at a rate of $$v_2$$ meters per jump. Given the starting locations and movement rates for each kangaroo, can you determine if they'll ever land at the same location at the same time?

Input Format

A single line of four space-separated integers denoting the respective values of $$x_1$$, $$v_1$$, $$x_2$$, and $$v_2$$.

Constraints

1. $$0 \le x_1 < x_2$$
2. $$1 \le v_1$$
3. $$1 \le v_2$$

Output Format

Print YES if they can land on the same location at the same time; otherwise, print NO.

Note: The two kangaroos must land at the same location after making the same number of jumps.

Sample Input 0

0 3 4 2

Sample Output 0

YES

Explanation 0

The two kangaroos jump through the following sequence of locations:

1. 0 3 6 9 12
2. 4 6 8 10 12

Thus, the kangaroos meet after 4 jumps and we print YES.

Sample Input 1

0 2 5 3

Sample Output 1

NO

Explanation 1

The second kangaroo has a starting location that is ahead (further to the right) of the first kangaroo's starting location (i.e., $$x_2 > x_1$$). Because the second kangaroo moves at a faster rate (meaning $$v_2 > v_1$$) and is already ahead of the first kangaroo, the first kangaroo will never be able to catch up. Thus, we print NO.

• What is the source of this puzzle? Dec 13 '18 at 15:09

$$v_1 > v_2$$ (so that kangaroo 1 catches up)
$$v_1 - v_2 | x_2 - x_1$$, here | means 'is a divisor of'.
After $$n \in \Bbb{N}$$ jumps, kangaroo 1 will be at position $$x_1 + n v_1$$ and kangaroo 2 at $$x_2 + n v_2$$. Now, if $$x_1 + n v_1 = x_2 + n v_2$$ $$n v_1 - n v_2 = x_2 - x_1$$ $$n (v_1 - v_2) = x_2 - x_1$$ $$n = \frac{x_2 - x_1}{v_1 - v_2}$$ This fraction is an integer if and only if $$v_1 - v_2$$ divides $$x_2 - x_1$$.