lis = [float(x) for x in input().split()]
target = float(input())

def binary(lis,target):
    lo = 0;
    hi = 100
    m = ( lo + hi ) /2.0
    while hi-lo>0.000001:

        m = (lo+hi)/2.0

        total = calculate(lis+[m])

        if total <= target+0.00001:
            lo = m
        else:
            hi = m
    return m

def calculate(lis):
    #print(lis)
    return sum(sorted(lis)[1:4])/3.0


result = binary(lis,target)

if result <= 0.9:
    print("impossible")
elif result >= 30:
    print("infinite")
else:
    print("%.2f" % result)