#include <fstream>

using namespace std;

ifstream in("numarare.in");
ofstream out("numarare.out");

int n,nr1,nr,i,j;
int v[1000000];

int simetric(int inceput,int sfarsit)
{
    if((sfarsit-inceput)%2==1) return 0;
    while(inceput<sfarsit)
    {
        if(v[inceput]!=v[sfarsit])
            return 0;
        inceput++;
        sfarsit--;
    }
    return 1;
}
int main()
{
    in>>n;
    in>>nr1;
    for(i=1;i<n;i++)
    {
        in>>nr;
        v[i]=nr1-nr;
        nr1=nr;
    }
    int sol = n-1;
    i=n-1;
    while(i>2)
    {

        for(j=1;j<i;j++)
        if(simetric(j,i)==1)
        {
            sol +=(i-j)/2;
            i=j;
        }
        else i--;
    }
    out<<sol;
    return 0;
}