#nullable enable

#region
var (_input, _iter) = (Array.Empty<string>(), 0);
T I<T>() where T : IParsable<T>
{
    while (_iter >= _input.Length) (_input, _iter) = (Console.ReadLine()!.Split(' '), 0);
    return T.Parse(_input[_iter++], null);
}
#endregion

static T[] Range<T>(int n, Func<T> F) => Enumerable.Range(0, n).Select(_ => F()).ToArray();

var n = I<int>();
var az = Range(n, I<int>);

var rz = az.AsSpan().ToArray();
Array.Reverse(rz);

var oz = new int[n];
oz.AsSpan().Fill(1);
var ha = new RollingHash(az);
var hr = new RollingHash(rz);
var ho = new RollingHash(oz);

var ans = 0L;

for (var i = 0; i < n; i++)
{
    var s = az[i] * 2;
    var j = n - 1 - i;
    var pass = 1;
    var fail = Math.Min(i, j) + 2;
    while (Math.Abs(pass - fail) > 1)
    {
        var mid = (pass + fail) >> 1;
        var h = RollingHash.Mod(ha.Slice(i, mid) + hr.Slice(j, mid));
        if (h == RollingHash.Mul(ho.Slice(0, mid), s)) pass = mid;
        else fail = mid;
    }
    ans += pass;
}
for (var i = 1; i < n; i++)
{
    var s = az[i] + az[i - 1];
    var j = n - i;
    var pass = 1;
    var fail = Math.Min(i, j) + 1;
    while (Math.Abs(pass - fail) > 1)
    {
        var mid = (pass + fail) >> 1;
        var h = RollingHash.Mod(ha.Slice(i, mid) + hr.Slice(j, mid));
        if (h == RollingHash.Mul(ho.Slice(0, mid), s)) pass = mid;
        else fail = mid;
    }
    ans += pass;
}
Console.WriteLine(ans);

class RollingHash
{
    static readonly long b = new Random().NextInt64(M61 >> 1, M61);
    static long[] Digit { get; set; } = new long[]{ 1 };
    long[] Hash { get; init; }
    public int[] String { get; init; }

    public RollingHash(int[] s)
    {
        var n = s.Length;
        Resize(n + 1);
        var hash = new long[n + 1];
        for (var i = 1; i <= n; i++) hash[i] = Mod(Mul(hash[i - 1], b) + s[i - 1]);
        (String, Hash) = (s, hash);
    }

    public long Slice(int begin) => Slice(begin, Length - begin);
    public long Slice(int begin, int length) => Mod(Hash[begin + length] - Mul(Hash[begin], Digit[length]));

    public int Length { get => Hash.Length - 1; }

    static void Resize(int size)
    {
        var l = Digit.Length;
        if (l >= size) return;
        while (l < size) l <<= 1;
        var d = new long[l];
        d[0] = 1;
        for (var i = 1; i < l; i++) d[i] = Mod(Mul(d[i - 1], b));
        Digit = d;
    }

    const long M61 = (1L << 61) - 1;
    const long M31 = (1L << 31) - 1;
    const long M30 = (1L << 30) - 1;

    public static long Mul(long x, long y)
    {
        long xu = x >> 31;
        long xd = x & M31;
        long yu = y >> 31;
        long yd = y & M31;
        long head = xu * yu * 2;
        long mid = xd * yu + xu * yd;
        long midu = mid >> 30;
        long midd = mid & M30;
        long last = Mod(xd * yd);
        return Mod(head + midu + (midd << 31) + last);
    }

    public static long Mod(long x)
    {
        while (x < 0) x += M61;
        long y = (x >> 61) + (x & M61);
        if (y >= M61) y -= M61;
        return y;
    }
}