def manacher(x):
    n=len(x)
    res=[0]*n
    i,j=0,0
    while i<n:
        while i-j>=0 and i+j<n and x[i-j]==x[i+j]:
            j+=1
        res[i]=j;k=1
        while i-k>=0 and k+res[i-k]<j:
            res[i+k]=res[i-k];k+=1
        i+=k;j-=k
    return res

n=int(input());ans=n
a=list(map(int,input().split()))
x=[]
for i in range(1,n):
    x.append(a[i]-a[i-1]);x.append(1<<60)
x=manacher(x[:-1])
for i in range(len(x)):
    if i&1:
        ans+=x[i]//2
    else:
        ans+=(x[i]+1)//2
print(ans)