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 print(ans)