Inclusion probability under a two-stage unequal-then-equal design

Let N = 4. Two units are selected for inclusion in the sample based on the following design. The $i$-th unit is selected with probability $p_i$ where $p_1=1/6, p_2=1/6, p_3=2/3$. The second unit is selected randomly from among the remaining three units in the population. Then, the first unit is included in the sample with probability: