using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using static System.Math;
using System.Text;
using System.Threading;
using System.Globalization;
using System.Runtime.CompilerServices;
using Library;

namespace Program
{
    public static class ABC149E
    {
        static public int numberOfRandomCases = 0;
        static public void MakeTestCase(List<string> _input, List<string> _output, ref Func<string[], bool> _outputChecker)
        {
        }
        static public void Solve()
        {
            var N = NN;
            var M = NN;
            var AList = NNList(N).OrderByRand().OrderBy(e => e).ToArray();
            var tree = new LIB_RedBlackTree<long>();
            foreach (var item in AList)
            {
                tree.Add(item);
            }
            var left = 2L;
            var right = 200001L;
            while (right - left > 1)
            {
                var mid = (right + left) / 2;
                var cnt = 0L;
                foreach (var item in AList)
                {
                    var tgt = mid - item;
                    var idx = tree.LowerBound(tgt);
                    cnt += N - idx;
                }
                if (cnt >= M) left = mid;
                else right = mid;
            }
            var pivot = left;
            var seg = new LIB_DualSegTree<long>(N, 0, (x, y) => x + y);
            for (var i = 0; i < N; i++)
            {
                var idx = tree.LowerBound(pivot - AList[i]);
                if (idx != N) seg.Update(idx, N, 1);
                seg[i] += N - idx;
            }
            var ans = 0L;
            var count = 0L;
            for (var i = 0; i < N; i++)
            {
                ans += seg[i] * AList[i];
                count += seg[i];
            }
            if (count / 2 > M) ans -= pivot * (count / 2 - M);
            Console.WriteLine(ans);
        }
        static class Console_
        {
            static Queue<string> param = new Queue<string>();
            public static string NextString() { if (param.Count == 0) foreach (var item in Console.ReadLine().Split(' ')) param.Enqueue(item); return param.Dequeue(); }
        }
        class Printer : StreamWriter
        {
            public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
            public Printer(Stream stream) : base(stream, new UTF8Encoding(false, true)) { base.AutoFlush = false; }
            public Printer(Stream stream, Encoding encoding) : base(stream, encoding) { base.AutoFlush = false; }
        }
        static public void Main(string[] args) { if (args.Length == 0) { Console.SetOut(new Printer(Console.OpenStandardOutput())); } var t = new Thread(Solve, 134217728); t.Start(); t.Join(); Console.Out.Flush(); }
        static long NN => long.Parse(Console_.NextString());
        static double ND => double.Parse(Console_.NextString());
        static string NS => Console_.NextString();
        static long[] NNList(long N) => Repeat(0, N).Select(_ => NN).ToArray();
        static double[] NDList(long N) => Repeat(0, N).Select(_ => ND).ToArray();
        static string[] NSList(long N) => Repeat(0, N).Select(_ => NS).ToArray();
        static IEnumerable<T> OrderByRand<T>(this IEnumerable<T> x) => x.OrderBy(_ => xorshift);
        static long Count<T>(this IEnumerable<T> x, Func<T, bool> pred) => Enumerable.Count(x, pred);
        static IEnumerable<T> Repeat<T>(T v, long n) => Enumerable.Repeat<T>(v, (int)n);
        static IEnumerable<int> Range(long s, long c) => Enumerable.Range((int)s, (int)c);
        static uint xorshift { get { _xsi.MoveNext(); return _xsi.Current; } }
        static IEnumerator<uint> _xsi = _xsc();
        static IEnumerator<uint> _xsc() { uint x = 123456789, y = 362436069, z = 521288629, w = (uint)(DateTime.Now.Ticks & 0xffffffff); while (true) { var t = x ^ (x << 11); x = y; y = z; z = w; w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)); yield return w; } }
    }
}
namespace Library {
    class LIB_RedBlackTree
    {
        public struct SumEntity
        {
            public long s;
            public long c;
        }
        static public LIB_RedBlackTree<long, long, long> CreateRUQRmQ() => new LIB_RedBlackTree<long, long, long>(long.MaxValue, long.MaxValue, Math.Min, (x, y) => y, (x, y) => y);
        static public LIB_RedBlackTree<long, long, long> CreateRAQRmQ() => new LIB_RedBlackTree<long, long, long>(long.MaxValue, 0, Math.Min, (x, y) => x + y, (x, y) => x + y);
        static public LIB_RedBlackTree<long, long, long> CreateRUQRMQ() => new LIB_RedBlackTree<long, long, long>(long.MinValue, long.MinValue, Math.Max, (x, y) => y, (x, y) => y);
        static public LIB_RedBlackTree<long, long, long> CreateRAQRMQ() => new LIB_RedBlackTree<long, long, long>(long.MinValue, 0, Math.Max, (x, y) => x + y, (x, y) => x + y);
        static public LIB_RedBlackTree<long, SumEntity, long> CreateRUQRSQ() => new LIB_RedBlackTree<long, SumEntity, long>(new SumEntity { c = 0, s = 0 }, long.MaxValue, (x, y) => new SumEntity { c = x.c + y.c, s = x.s + y.s }, (x, y) => new SumEntity { c = x.c, s = x.c * y }, (x, y) => y);
        static public LIB_RedBlackTree<long, SumEntity, long> CreateRAQRSQ() => new LIB_RedBlackTree<long, SumEntity, long>(new SumEntity { c = 0, s = 0 }, 0, (x, y) => new SumEntity { c = x.c + y.c, s = x.s + y.s }, (x, y) => new SumEntity { c = x.c, s = x.s + x.c * y }, (x, y) => x + y);
    }
    class LIB_RedBlackTree<Key, ValueT, ValueE>
    {
        bool ope;
        class Node
        {
            public Node left;
            public Node right;
            public Key key;
            public ValueT val;
            public ValueT dat;
            public ValueE lazy;
            public bool isBlack;
            public int cnt;
            public bool needRecalc;
        }
        Func<ValueT, ValueT, ValueT> f;
        Func<ValueT, ValueE, ValueT> g;
        Func<ValueE, ValueE, ValueE> h;
        ValueT ti;
        ValueE ei;
        Comparison<Key> c;
        Node root;
        bool isNeedFix;
        Node lmax;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_RedBlackTree(ValueT ti, ValueE ei, Func<ValueT, ValueT, ValueT> f, Func<ValueT, ValueE, ValueT> g, Func<ValueE, ValueE, ValueE> h, Comparison<Key> c, bool ope = true)
        {
            this.ti = ti;
            this.ei = ei;
            this.f = f;
            this.g = g;
            this.h = h;
            this.c = c;
            this.ope = ope;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_RedBlackTree(ValueT ti, ValueE ei, Func<ValueT, ValueT, ValueT> f, Func<ValueT, ValueE, ValueT> g, Func<ValueE, ValueE, ValueE> h) : this(ti, ei, f, g, h, Comparer<Key>.Default.Compare) { }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        bool IsRed(Node n) => n != null && !n.isBlack;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        bool IsBlack(Node n) => n != null && n.isBlack;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        int Cnt(Node n) => n?.cnt ?? 0;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        void Eval(Node n)
        {
            if (n == null || ei.Equals(n.lazy)) return;
            n.val = g(n.val, n.lazy);
            if (!n.needRecalc) n.dat = g(n.dat, n.lazy);
            if (n.left != null) n.left.lazy = h(n.left.lazy, n.lazy);
            if (n.right != null) n.right.lazy = h(n.right.lazy, n.lazy);
            n.lazy = ei;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        void Recalc(Node n)
        {
            if (n == null) return;
            Eval(n);
            if (!n.needRecalc) return;
            n.needRecalc = false;
            n.dat = n.val;
            if (n.left != null)
            {
                Recalc(n.left);
                n.dat = f(n.left.dat, n.dat);
            }
            if (n.right != null)
            {
                Recalc(n.right);
                n.dat = f(n.dat, n.right.dat);
            }
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node RotateL(Node n)
        {
            if (ope) { Eval(n); Eval(n.right); }
            Node m = n.right, t = m.left;
            m.left = n; n.right = t;
            n.cnt -= m.cnt - Cnt(t);
            m.cnt += n.cnt - Cnt(t);
            if (ope) { n.needRecalc = true; m.needRecalc = true; }
            return m;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node RotateR(Node n)
        {
            if (ope) { Eval(n); Eval(n.left); }
            Node m = n.left, t = m.right;
            m.right = n; n.left = t;
            n.cnt -= m.cnt - Cnt(t);
            m.cnt += n.cnt - Cnt(t);
            if (ope) { n.needRecalc = true; m.needRecalc = true; }
            return m;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node RotateLR(Node n)
        {
            n.left = RotateL(n.left);
            return RotateR(n);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node RotateRL(Node n)
        {
            n.right = RotateR(n.right);
            return RotateL(n);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Add(Key key, ValueT val)
        {
            root = Add(root, key, val);
            root.isBlack = true;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node Add(Node n, Key key, ValueT val)
        {
            if (n == null)
            {
                isNeedFix = true;
                return new Node() { key = key, val = val, dat = val, lazy = ei, cnt = 1 };
            }
            if (ope) Eval(n);
            if (c(key, n.key) < 0) n.left = Add(n.left, key, val);
            else n.right = Add(n.right, key, val);
            if (ope) n.needRecalc = true;
            n.cnt++;
            return Balance(n);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node Balance(Node n)
        {
            if (!isNeedFix) return n;
            if (!IsBlack(n)) return n;
            if (IsRed(n.left) && IsRed(n.left.left))
            {
                n = RotateR(n);
                n.left.isBlack = true;
            }
            else if (IsRed(n.left) && IsRed(n.left.right))
            {
                n = RotateLR(n);
                n.left.isBlack = true;
            }
            else if (IsRed(n.right) && IsRed(n.right.left))
            {
                n = RotateRL(n);
                n.right.isBlack = true;
            }
            else if (IsRed(n.right) && IsRed(n.right.right))
            {
                n = RotateL(n);
                n.right.isBlack = true;
            }
            else isNeedFix = false;
            return n;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Remove(Key key)
        {
            root = Remove(root, key);
            if (root != null) root.isBlack = true;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node Remove(Node n, Key key)
        {
            if (n == null)
            {
                isNeedFix = false;
                return n;
            }
            if (ope) Eval(n);
            n.cnt--;
            var r = c(key, n.key);
            if (r < 0)
            {
                n.left = Remove(n.left, key);
                if (ope) n.needRecalc = true;
                return BalanceL(n);
            }
            if (r > 0)
            {
                n.right = Remove(n.right, key);
                if (ope) n.needRecalc = true;
                return BalanceR(n);
            }
            if (n.left == null)
            {
                isNeedFix = n.isBlack;
                return n.right;
            }
            n.left = RemoveMax(n.left);
            n.key = lmax.key;
            n.val = lmax.val;
            if (ope) n.needRecalc = true;
            return BalanceL(n);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node RemoveMax(Node n)
        {
            if (ope) Eval(n);
            n.cnt--;
            if (n.right != null)
            {
                n.right = RemoveMax(n.right);
                if (ope) n.needRecalc = true;
                return BalanceR(n);
            }
            lmax = n;
            isNeedFix = n.isBlack;
            return n.left;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node BalanceL(Node n)
        {
            if (!isNeedFix) return n;
            if (IsBlack(n.right) && IsRed(n.right.left))
            {
                var b = n.isBlack;
                n = RotateRL(n);
                n.isBlack = b;
                n.left.isBlack = true;
                isNeedFix = false;
            }
            else if (IsBlack(n.right) && IsRed(n.right.right))
            {
                var b = n.isBlack;
                n = RotateL(n);
                n.isBlack = b;
                n.right.isBlack = true;
                n.left.isBlack = true;
                isNeedFix = false;
            }
            else if (IsBlack(n.right))
            {
                isNeedFix = n.isBlack;
                n.isBlack = true;
                n.right.isBlack = false;
            }
            else
            {
                n = RotateL(n);
                n.isBlack = true;
                n.left.isBlack = false;
                n.left = BalanceL(n.left);
                isNeedFix = false;
            }
            return n;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        Node BalanceR(Node n)
        {
            if (!isNeedFix) return n;
            if (IsBlack(n.left) && IsRed(n.left.right))
            {
                var b = n.isBlack;
                n = RotateLR(n);
                n.isBlack = b; n.right.isBlack = true;
                isNeedFix = false;
            }
            else if (IsBlack(n.left) && IsRed(n.left.left))
            {
                var b = n.isBlack;
                n = RotateR(n);
                n.isBlack = b;
                n.left.isBlack = true;
                n.right.isBlack = true;
                isNeedFix = false;
            }
            else if (IsBlack(n.left))
            {
                isNeedFix = n.isBlack;
                n.isBlack = true;
                n.left.isBlack = false;
            }
            else
            {
                n = RotateR(n);
                n.isBlack = true;
                n.right.isBlack = false;
                n.right = BalanceR(n.right);
                isNeedFix = false;
            }
            return n;
        }
        public KeyValuePair<Key, ValueT> this[long i]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get { return At(root, i); }
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        KeyValuePair<Key, ValueT> At(Node n, long i)
        {
            if (ope) Eval(n);
            if (n.left == null)
            {
                if (i == 0) return new KeyValuePair<Key, ValueT>(n.key, n.val);
                else return At(n.right, i - 1);
            }
            if (n.left.cnt == i) return new KeyValuePair<Key, ValueT>(n.key, n.val);
            if (n.left.cnt > i) return At(n.left, i);
            return At(n.right, i - n.left.cnt - 1);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool Have(Key key)
        {
            var t = LowerBound(key);
            return t < Cnt(root) && c(At(root, t).Key, key) == 0;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public long UpperBound(Key key) => UpperBound(root, key);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        long UpperBound(Node n, Key key)
        {
            if (n == null) return 0;
            var r = c(key, n.key);
            if (r < 0) return UpperBound(n.left, key);
            return Cnt(n.left) + 1 + UpperBound(n.right, key);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public long LowerBound(Key key) => LowerBound(root, key);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        long LowerBound(Node n, Key key)
        {
            if (n == null) return 0;
            var r = c(key, n.key);
            if (r <= 0) return LowerBound(n.left, key);
            return Cnt(n.left) + 1 + LowerBound(n.right, key);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public KeyValuePair<Key, ValueT> Min()
        {
            Node n = root, p = null;
            while (n != null)
            {
                Eval(p = n);
                n = n.left;
            }
            return new KeyValuePair<Key, ValueT>(n.key, n.val);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public KeyValuePair<Key, ValueT> Max()
        {
            Node n = root, p = null;
            while (n != null)
            {
                Eval(p = n);
                n = n.right;
            }
            return new KeyValuePair<Key, ValueT>(n.key, n.val);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Update(long l, long r, ValueE val) => Update(root, l, r, val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        void Update(Node n, long l, long r, ValueE val)
        {
            if (n == null) return;
            Eval(n);
            n.needRecalc = true;
            var lc = Cnt(n.left);
            if (lc < l) Update(n.right, l - lc - 1, r - lc - 1, val);
            else if (r <= lc) Update(n.left, l, r, val);
            else if (l <= 0 && Cnt(n) <= r) n.lazy = val;
            else
            {
                n.val = g(n.val, val);
                if (l < lc) Update(n.left, l, lc, val);
                if (lc + 1 < r) Update(n.right, 0, r - lc - 1, val);
            }
            return;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public ValueT Query(long l, long r) => Query(root, l, r);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        ValueT Query(Node n, long l, long r)
        {
            var v1 = ti; var v2 = ti; var v3 = ti;
            if (n == null) return ti;
            Eval(n);
            var lc = Cnt(n.left);
            if (lc < l) v3 = Query(n.right, l - lc - 1, r - lc - 1);
            else if (r <= lc) v1 = Query(n.left, l, r);
            else if (l <= 0 && Cnt(n) <= r)
            {
                Recalc(n);
                v2 = n.dat;
            }
            else
            {
                if (l < lc) v1 = Query(n.left, l, lc);
                if (lc + 1 < r) v3 = Query(n.right, 0, r - lc - 1);
                v2 = n.val;
            }
            return f(f(v1, v2), v3);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool Any() => root != null;
        public long Count => Cnt(root);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public IEnumerable<KeyValuePair<Key, ValueT>> List() => L(root);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        IEnumerable<KeyValuePair<Key, ValueT>> L(Node n)
        {
            if (n == null) yield break;
            foreach (var i in L(n.left)) yield return i;
            yield return new KeyValuePair<Key, ValueT>(n.key, n.val);
            foreach (var i in L(n.right)) yield return i;
        }
    }
    class LIB_RedBlackTree<Key, Value>
    {
        LIB_RedBlackTree<Key, Value, Value> tree;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_RedBlackTree(Comparison<Key> c) { tree = new LIB_RedBlackTree<Key, Value, Value>(default(Value), default(Value), (x, y) => x, (x, y) => x, (x, y) => x, c, false); }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_RedBlackTree() : this(Comparer<Key>.Default.Compare) { }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Add(Key key, Value val) => tree.Add(key, val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Remove(Key key) => tree.Remove(key);
        public KeyValuePair<Key, Value> this[long i]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get { return tree[i]; }
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool Have(Key key) => tree.Have(key);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public long UpperBound(Key key) => tree.UpperBound(key);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public long LowerBound(Key key) => tree.LowerBound(key);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public KeyValuePair<Key, Value> Min() => tree.Min();
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public KeyValuePair<Key, Value> Max() => tree.Max();
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool Any() => tree.Any();
        public long Count => tree.Count;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public IEnumerable<KeyValuePair<Key, Value>> List() => tree.List();
    }
    class LIB_RedBlackTree<T>
    {
        LIB_RedBlackTree<T, T, T> tree;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_RedBlackTree(Comparison<T> c) { tree = new LIB_RedBlackTree<T, T, T>(default(T), default(T), (x, y) => x, (x, y) => x, (x, y) => x, c, false); }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_RedBlackTree() : this(Comparer<T>.Default.Compare) { }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Add(T val) => tree.Add(val, val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Remove(T val) => tree.Remove(val);
        public T this[long i]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get { return tree[i].Key; }
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool Have(T val) => tree.Have(val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public long UpperBound(T val) => tree.UpperBound(val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public long LowerBound(T val) => tree.LowerBound(val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T Min() => tree.Min().Key;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T Max() => tree.Max().Key;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool Any() => tree.Any();
        public long Count => tree.Count;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public IEnumerable<T> List() => tree.List().Select(e => e.Key);
    }
    class LIB_DualSegTree<T>
    {
        int n, height;
        T ti;
        Func<T, T, T> f;
        T[] dat;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_DualSegTree(long _n, T _ti, Func<T, T, T> _f)
        {
            n = 1; height = 0;
            while (n < _n) { n <<= 1; ++height; }
            ti = _ti;
            f = _f;
            dat = Enumerable.Repeat(ti, n << 1).ToArray();
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_DualSegTree(IEnumerable<T> l, T _ti, Func<T, T, T> _f) : this(l.Count(), _ti, _f)
        {
            var idx = 0;
            foreach (var item in l) dat[n + idx++] = item;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        void Eval(long i)
        {
            if (dat[i].Equals(ti)) return;
            dat[(i << 1) | 0] = f(dat[(i << 1) | 0], dat[i]);
            dat[(i << 1) | 1] = f(dat[(i << 1) | 1], dat[i]);
            dat[i] = ti;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        void Thrust(long i)
        {
            for (var j = height; j > 0; j--) Eval(i >> j);
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        void Thrust(long l, long r)
        {
            if (l == r) { Thrust(l); return; }
            var xor = l ^ r;
            var i = height;
            for (; (xor >> i) != 0; --i) Eval(l >> i);
            for (; i != 0; --i) { Eval(l >> i); Eval(r >> i); }
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Update(long l, long r, T v)
        {
            if (r < l) throw new Exception();
            Thrust(l += n, r += n - 1);
            for (long li = l, ri = r + 1; li < ri; li >>= 1, ri >>= 1)
            {
                if ((li & 1) == 1) { dat[li] = f(dat[li], v); ++li; }
                if ((ri & 1) == 1) { --ri; dat[ri] = f(dat[ri], v); }
            }
        }
        public T this[long idx]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get { Thrust(idx += n); return dat[idx]; }
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            set { Thrust(idx += n); dat[idx] = value; }
        }
    }
}


Measured memory_bytes: 44436