#nullable enable

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

var n = I<int>();
var cards = new long[4];
for (var i = 0; i < n; i++)
{
    var a = I<int>();
    var b = I<int>();
    var k = 0;
    for (var j = 0; j < 3; j++)
    {
        if (a % 2 == 0)
        {
            a /= 2;
            k++;
        }
    }
    cards[k] += b;
}
var ans = cards[3];
var m = Math.Min(cards[2], cards[1]);
ans += m;
cards[2] -= m;
cards[1] -= m;
if (cards[2] > 0) ans += cards[2] / 2;
else ans += cards[1] / 3;
Console.WriteLine(ans);