#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
#define inf 1000000000
vector <int> v;
int n,i,nr,mij,dr,mij2,dr2,equiv,dp[200015],k;
int main(void)
{
    FILE * f;
    f=fopen("numarare.in","r");
    ofstream g("numarare.out");
    fscanf(f,"%d",&n);
    v.resize(200015);
    v[0]=inf;
    v[1]=-1;
    //v.push_back(inf);
    //v.push_back(-1);
    for (i=1;i<=n;i++)
    {
        fscanf(f,"%d",&nr);
        v[2*i]=nr;
        //v.push_back(nr);
        v[2*i+1]=-1;
        //v.push_back(-1);
    }
    v[2*n+2]=inf;
    //v.push_back(inf);
    mij=dr=-1;
    for (i=3;i<2*n;i=i+2)
    {
        if (i>=mij+dr)
        {
            mij=i;
            dr=1;
            while (v[mij+dr+2]+v[mij-dr-2]==v[mij+dr]+v[mij-dr])
                dr=dr+2;
            dp[i]=dr;
        }
        else
        {
            equiv=mij-(i-mij);
            mij2=i;
            dr2=min(dr-(i-mij),dp[equiv]);
            while (v[mij2+dr2+2]+v[mij2-dr2-2]==v[mij2+dr2]+v[mij2-dr2])
                dr2=dr2+2;
            dp[i]=dr2;
            if (dr2>dr)
            {
                dr=dr2;
                mij=mij2;
            }
        }
        k=k+(dp[i]+1)/2;
    }
    g<<k<<'\n';
    g.close();
    return 0;
}