N,M = map(int,input().split())
L = [int(input()) for i in range(int(input()))]
ans = 0
S = 1
E = 1+M-1
for i in L:
if S <= i <= E:
continue
else:
if i < S:
k = S-i
ans += k
S -= k
E -= k
else:
k = i - E
ans += k
S += k
E += k
print(ans)