What is the nth term for - 1,0,3,8,15 ...

From the sequence given:

-1,0,3,8,15 ...

the difference between the terms is:

1,3,5,7, ...

the above numbers are add numbers, thus the series if obtain by adding add number to the terms. Thus the formula will be:

an=a (n-1) + i

where n is the nth term

a1=-1

i takes all the off integers, i=1,3,5, ...