count and restart formula with two elements

i need a formula to count and restart counting when other element comes
example

1 1 1 2 2 2 2 2 1 2 1 1 1 1 1
1 2 3 1 2 3 4 5 1 1 1 2 3 4 5

if first 1 is at A1 which is the formula at A2?