import math

n, x, m = [int(t.strip()) for t in input().strip().split()]
circle = [0]*(n+1)
for _ in range(m):
    c = int(input().strip())
    circle[c] += 1
    
clockwise_crates = 0
r = 1
while r!=x:
    clockwise_crates += circle[r]
    r += 1

counter_crates = 0
if x>1:
    r = n
    counter_crates = circle[n]
    while r!=x:
        r -= 1
        counter_crates += circle[r]
        
print( min( clockwise_crates, counter_crates ) )