My Question is that what will be the effect of number of iteration in Robin miller algo

sometimes it produce correct answer after passing 0 or 1 iteration sometimes it need upto 8 iteration

whats the reason ??

i got AC using 1 iteration in PON (spoj)

while i got AC using 8 iteration in http://www.codechef.com/MAY13/problems/WITMATH(may challenge)

here is the algorithm

Algorithm

Input :A number N to be tested and a variable iteration-the number

of ‘a’ for which algorithm will test N.

Output :0 if N is definitely a composite and 1 if N is probably a prime.

Write N as

For each iteration

Pick a random a in [1,N-1]

x = mod n

if x =1 or x = n-1

Next iteration

for r = 1 to s-1

x = mod n

if x = 1

return false

if x = N-1

Next iteration

return false

return true