N=int(input())
list1=[]
line=input()
line=line.rstrip().split(" ")
for i in range(N):
    list1.append(int(line[i]))
    
#print(list1)

count=0
for j in range(1,N):
    if list1[j-1]<list1[j]:
        pass
    else:
        list1[j]=list1[j-1]+1
        count+=1
print(count)