What is vertical position in pendulum?

The simple pendulum of a tall clock swings freely backwards and forwards so that it passes through the vertical position at 1second intervals.

Assuming that the motion of the pendulum is simple harmonic, what is the length?

[Take the gravitational field strength to be 10N/kg, and take (π)^2 to be 10.]

And I want to know how to solve… The answer is 1m.