import sys
import pypyjit
import itertools
import heapq
import math
from collections import deque, defaultdict
import bisect

input = sys.stdin.readline
sys.setrecursionlimit(10 ** 6)
pypyjit.set_param('max_unroll_recursion=-1')


def index_lt(a, x):
    'return largest index s.t. A[i] < x or -1 if it does not exist'
    return bisect.bisect_left(a, x) - 1


def index_le(a, x):
    'return largest index s.t. A[i] <= x or -1 if it does not exist'
    return bisect.bisect_right(a, x) - 1


def index_gt(a, x):
    'return smallest index s.t. A[i] > x or len(a) if it does not exist'
    return bisect.bisect_right(a, x)


def index_ge(a, x):
    'return smallest index s.t. A[i] >= x or len(a) if it does not exist'
    return bisect.bisect_left(a, x)


N = int(input())
B = list(map(int, input().split()))

h = []
for i in range(N):
    if B[i] > 0:
        heapq.heappush(h, i)
ans = 0
for i in range(N):
    if B[i] < 1:
        j = h[0]
        ans += j - i
        B[i] += 1
        B[j] -= 1
        if B[j] == 0:
            heapq.heappop(h)
    if B[i] > 1:
        ans += B[i] - 1
        B[i + 1] += B[i] - 1
        B[i] = 1
    if len(h) > 0 and h[0] == i:
        heapq.heappop(h)
print(ans)