The synchronized phasor measurement unit (PMU) provides fast, precise, and synchronized measurements, which is crucial for power systems dynamic monitoring. However, replacing conventional SCADA systems with PMUs is happening gradually and slowly due to its considerable cost. Hence, with restricted budgets, the installation of these measuring devices should be selective. The majority of previous works on PMU sensor placement have been focused on steady-state estimation. Therefore, they concentrate on network topology to find optimal configurations making the whole network observable at steady-state. However, approaches based on steady-state or quasi-steady-state operating conditions are not applicable for power systems experiencing fast and dynamic changes. Moreover, a dynamic model for several variables and parts in a power system may be wholly or partly unknown. In such cases, the concept of partially known power systems as well as simultaneous input and state estimation can be exploited to obtain the estimates of the known system states and unknown inputs. In this work, we develop a greedy approach to obtain optimal sensor placement for joint input and state estimation of partially known power grids.