Consider two events $\mathcal{A}$ and $\mathcal{B}$ corresponding to the beggining and the ending of trajectories of two massive particles, respectively. The particle named $\mathcal{P1}$ is in free motion, and the other particle $\mathcal{P2}$ is in accelerated motion. Both particles measure events $\mathcal{A}$ and $\mathcal{B}$ as events that occurs at same place, in both rest-frames, so both particles also measure their respective proper times elapsed between these events. How can I prove that the proper time of the free particle $\mathcal{P1}$ is bigger than proper time of the accelerated particle $\mathcal{P2}$?