N=int(input())
L = list(map(int, input().split()))
for i in range(N):
    L[i] -=1
ans = 0
temp = 0
for i in range(N):
    if L[i]<0:
        for j in range(max(i+1,temp),N):
            if L[j]>0:
                L[i]=0
                L[j]-=1
                ans +=j-i
                temp = j
                break
    if L[i]>0:
        j = i+1
        while L[i]>0 and j<N:
            if L[j]<0:
                L[j]=0
                L[i]-=1
                ans +=j-i
            j+=1
print(ans)