# coding: utf-8
# Your code here!
N=int(input())

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

empty=[]
rem=[]

ans=0
for i in range(len(B)):
    if B[i]==0:
        if len(rem)==0:
            empty.append(i)
        else:
            ans+=(i-rem.pop(-1))
            
    else:
        while B[i]>1 and empty:
            ans+=(i-empty.pop(-1))
            B[i]-=1
        
        for j in range(B[i]-1):
            rem.append(i)
        
print(ans)