MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
input = sys.stdin.readline
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)
from bisect import bisect_left,bisect_right
from collections import deque

def main():
    n = int(input())
    b = list(map(int,input().split()))

    ans = 0
    idx1 = deque()
    idx2 = deque()
    for i in range(n):
        if b[i] == 1:
            continue
        elif b[i] == 0:
            if len(idx2) > 0:
                p = idx2.popleft()
                ans += i - p 
            else:
                idx1.append(i)
        elif b[i] > 1:
            while len(idx1) and b[i] > 1:
                b[i] -= 1
                p = idx1.popleft()
                ans += i - p
            
            for _ in range(b[i] - 1):
                idx2.append(i)
    print(ans)

if __name__ =='__main__':
    main()