Projectile motion in electric field

In the above picture, a charged particle enters a uniform electric field at the point $O$ in the direction perpendicular to the electric field, and moves along the blue dotted projectile path $O \rightarrow P \rightarrow Q \rightarrow$ . Now, suppose that it takes the same amount of time for the particle to move from $O$ to $P$ and from $P$ to $Q.$ Also suppose that the potential difference between $O$ and $P$ is $V.$ Then what is the potential difference between $P$ and $Q?$ (Ignore the gravitational force.)