Four branches of a company are located at $M, N, O$ and $P, M$ is north of $N$ at a distance of $4$ km; $P$ is south of $O$ at a distance of $2$ km; $N$ is southeast of $O$ by $1$ km. What is the distance between $M$ and $P$ in km?
1. $5.34$
2. $6.74$
3. $28.5$
4. $45.49$