My question is, how can I calculate the input impedance of this circuit? I want to find the expression, that describes the input impedance.
You've posted a Sallen-Key Band-Pass Filter. Personally I'd avoid doing the math -- The way I would approach it is to build the circuit in LTSpice then plot V(in)/I(in) vs. Frequency to get a plot of the input impedance vs. frequency.
That said, you can gain some insight by considering extreme cases when
As f->inf, you're going to see R1 looking in as C shorts to ground. As f->DC your C's are like open-circuits. The V+ terminal of the op-amp will be pulled to ground through R2 so the output of the op-amp will be 0V, so it will be like R3 is connected to ground. Thus, looking in at DC you'll see R1 + R3.