#nullable enable

using System.Numerics;

object? Solve()
{
    var s = String();
    var rle = s.RunLengthEncoding();
    var ans = 0L;
    var i = 0;
    if (rle[0].Item1 == '0') i++;
    var s1 = 0L;
    while (i < rle.Count)
    {
        var (c, k) = rle[i];
        if (c == '0')
        {
            if (s1 % 2 == 1) s1--;
            ans += s1 / 2 * k;
        }
        else
        {
            s1 += k;
        }
        i++;
    }
    return ans;
}

#region
AtCoderIO _io_;
var _backend_ = new StandardIOBackend();
_io_ = new(){ Backend = _backend_ };
Out(Solve());
_backend_.Flush();

string String() => _io_.Next();
int Int() => int.Parse(String());
void Out(object? x, string? sep = null) => _io_.Out(x, sep ?? Environment.NewLine);

class AtCoderIO
{
    public required StandardIOBackend Backend { get; init; }

    Memory<string> _input = Array.Empty<string>();
    int _iter = 0;
    public string Next()
    {
        while (_iter >= _input.Length) (_input, _iter) = (Backend.ReadLine().Trim().Split(' '), 0);
        return _input.Span[_iter++];
    }

    public void Out(object? x, string separator)
    {
        if (x == null) return;
        if (x is System.Collections.IEnumerable a and not string)
        {
            var objects = a.Cast<object>();
            if (!objects.Any()) return;
            x = string.Join(separator, objects);
        }
        Backend.WriteLine(x);
    }
}

class StandardIOBackend
{
    readonly StreamReader _sr = new(Console.OpenStandardInput());
    readonly StreamWriter _sw = new(Console.OpenStandardOutput()) { AutoFlush = false };
    public string ReadLine() => _sr.ReadLine()!;
    public void WriteLine(object? value) => _sw.WriteLine(value);
    public void Flush() => _sw.Flush();
}
#endregion

static class Extensions
{
    public static T[] Repeat<T>(this int time, Func<T> F) => Enumerable.Range(0, time).Select(_ => F()).ToArray();

    public static List<(T, int)> RunLengthEncoding<T>(this IEnumerable<T> s) where T : IEquatable<T>
    {
        var res = new List<(T, int)>();
        if (!s.Any()) return res;
        var (pc, pl) = (s.First(), 1);
        foreach (var c in s.Skip(1))
        {
            if (!c.Equals(pc))
            {
                res.Add((pc, pl));
                (pc, pl) = (c, 0);
            }
            pl++;
        }
        res.Add((pc, pl));
        return res;
    }
}