n=int(input())
L=list(map(int,input().split()))
m,p,b=sum(L),1,1
while b<=m:p+=1;b+=2*p-1
A=list(range(1,p))
P=A+A[p-3::-1]
D=[a-l for a,l in zip(P,L)]
x=-sum([i for i in D if i<0])
y=sum([i for i in D if i>0])
print(max(x,y))