using System;
using System.Linq;
public class Test
{
public static void Main()
{
int[] a = {1, 2, 3, 4, 5, 6};
var res = a.Select((x, i) => new {x, i}).GroupBy(x => x.i >> 1, x=> x.x).Select(x => x.Sum());
Console.WriteLine(string.Join(" ", res));
}
}
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