a = int(input())
b = int(input())
c = int(input())
list = [a, b, c]
list.sort()

a = list[0]
b = list[1]
c = list[2]
diff = (a + b + c) // 3
sum = diff*2 - a - b
if (a + b + c) % 3 != 0:
    print("IMPOSSIBLE")
    import sys
    sys.exit()
print(sum)