2253.py

# 점프

# DP
import sys
input = sys.stdin.readline

N, M = map(int, input().split())
dp = [[float('inf')] * (int((2 * N) ** 0.5) + 2) for _ in range(N + 1)]
dp[1][0] = 0

stone_set = set()
for _ in range(M):
    stone_set.add(int(input()))
for i in range(2, N + 1):
    if i in stone_set:
        continue
    for j in range(1, int((2 * i) ** 0.5) + 1):
        dp[i][j] = min(dp[i - j][j - 1], dp[i - j][j], dp[i - j][j + 1]) + 1


if min(dp[N]) == float('inf'):
    print(-1)
else:
    print(min(dp[N]))

# BFS
# import sys
# from collections import deque

# N, M = map(int, sys.stdin.readline().split())
# check = [[] for _ in range(N + 1)]
# small_rock = set()
# for _ in range(M):
#     small = int(sys.stdin.readline())
#     small_rock.add(small)


# def solution(N, check, small_rock):
#     queue = deque([(1, 0, 0)])
#     while queue:
#         location, jump, n = queue.popleft()
#         for x in [jump + 1, jump, jump - 1]:
#             if x > 0:
#                 next_location = location + x
#                 if next_location == N:
#                     return n + 1
#                 if next_location < N and next_location not in small_rock and x not in check[next_location]:
#                     check[next_location].append(x)
#                     queue.append((next_location, x, n + 1))
#     return -1


# print(solution(N, check, small_rock))