Which is the input resistance of the second amplifier?
Which is the input resistance of the second amplifier?

It's a tad more complex than it first seems. The circuit operates as a precision full wave rectifier and the 2nd stage op-amp is summing two signals: -

The input signal via \$2R_2\$ and
An inverted half wave version of the input via \$R_2\$

A version of your circuit is here courtesy of Elliot Sound Products: -

As you can see the final stage sums a negative half-wave rectified signal with the original signal to make a full wave signal.