# The duel cowboys
# The probability A hits B is p_a = 3/4
# The probability B hits A is p_b = 1/3
# The duel ends when one hits the other
# A shoots first
# Compute the expected number of turns that the duel will end
# Exact answer, it is a geometric distribution with p = (3/4 + 1/4*1/3) = 5/6, E(1_Head) = 6/5 = 1.2 

# Alternatively, 1.(3/4 + 1/4.1/3) + 2.(1/4.2/3.3/4 + 1/4.2/3.1/4.1/3) ....
# = 1.5/6 + 2.(5/6^2) + 3.(5/6^3) + ... + n.(5/6^n) 
# Arithmetic-geometric sequence a = 1, b = 5/6, d = 1, r = 1/6
# t_n = [1 + (n-1).1].5/6.(1/6)^(n-1)
# limit of Sn = 5/6(6/5 + 6/25) = 1 + 1/5 = 1.2

# flag : 0: one is hit, otherwise 1
def f():
    var flag

    flag = 1
    while flag > 0:
        prob(3,1):
            flag = 0
        else:
            prob(1,2):
                flag = 0
            else:
                flag = 1
        tick 1