Sum of the series $sinθsin2θ + sin2θsin3θ + sin3θsin4θ + sin4θsin5θ + cdots+sin nthetasin(n+1)theta$ terms

The series is given:
$$\sum_{i=1}^n \sin (i\theta) \sin ((i+1)\theta)$$
We have to find the sum to n terms of the given series.
I could took out the $2\sin^2\cos$ terms common in the series. But what to do further, please guide me.