import sys, time, random
from collections import deque, Counter, defaultdict
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 63 - 1
mod = 998244353

def Dijkstra(s, graph):
    INF = 2 ** 63 - 1
    import heapq
    n = len(graph)
    dist = [INF] * n
    dist[s] = 0
    bef = [0] * n
    bef[s] = s
    hq = [(0, s)]
    heapq.heapify(hq)
    while hq:
        c, now = heapq.heappop(hq)
        
        if c > dist[now]:
            continue
        for to, cost in graph[now]:
            if dist[now] + cost < dist[to]:
                dist[to] = cost + dist[now]
                bef[to] = now
                heapq.heappush(hq, (dist[to], + to))
    return dist, bef

def DijkstraRest(bef, t):
    now = t
    ret = []
    while bef[now] != now:
        ret.append((bef[now], now))
        now = bef[now]
    ret.reverse()
    return ret

n = ii()

a = li()
graph = [[] for _ in range(n)]
for i in range(n):
    if 0 <= i - 1:
        graph[i].append((i - 1, abs(a[i] - a[i - 1]) - 1))
    if i + 1 < n:
        graph[i].append((i + 1, abs(a[i] - a[i + 1]) - 1))

d, _ = Dijkstra(0, graph)

print(d[n - 1] + a[0] - 1)