Formula For Returning The Next Value Unless Value In Adjacent Column In 1

The first two columns are an example of my data set; I would like a formula for the 3rd column (Length+1) which will give me the length at the next age (i.e the value in the cell below in most cases) unless the following age is 1 in which case I want it to return zero.

Age Length Length+1
1 22 51
2 51 78
3 78 94
4 94 0
1 31 46
2 46 69
3 69 85
4 85 96
5 96 103
6 103 0
1 19 36
2 36 52
3 52 0