3

s = int(input())
e = int(input())
n = int(input())
min_l = 10**9
min_r = min_l
for i in range(n):
    x = int(input())
    min_l = min(min_l, abs(x - s))
    min_r = min(min_r, abs(x - e))
print(min(abs(s - e), min_l + min_r + 1))