Problem:
Score : 300 points

Problem Statement

There is an empty array.
The following N operations will be performed to insert integers into the array.
In the i-th operation (1≤i≤N), b_i copies of an integer a_i are inserted into the array.
Find the K-th smallest integer in the array after the N operations.
For example, the 4-th smallest integer in the array \{1,2,2,3,3,3\} is 3.

Constraints

1≤N≤10^5

1≤a_i,b_i≤10^5

1≤K≤b_1…+…b_n

All input values are integers.

Input

Input is given from Standard Input in the following format:

N K
a_1 b_1
:
a_N b_N

Output

Print the K-th smallest integer in the array after the N operations.

Sample Input 1

3 4
1 1
2 2
3 3

Sample Output 1

3

The resulting array is the same as the one in the problem statement.

Sample Input 2

10 500000
1 100000
1 100000
1 100000
1 100000
1 100000
100000 100000
100000 100000
100000 100000
100000 100000
100000 100000

Sample Output 2

1

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
class Program
{
    static void Main(string[] args)
    {
            // 入力
            string[] sS = Console.ReadLine().Trim().Split(' ');
            int iN = int.Parse(sS[0]);
            long iK = long.Parse(sS[1]);
            List<ClsAtoB> lstX = new List<ClsAtoB>();
            for (int i = 0; i < iN; i++)
            {
                sS = Console.ReadLine().Trim().Split(' ');
                int ia = int.Parse(sS[0]);                              // 何が
                int ib = int.Parse(sS[1]);                              // 何個
                lstX.Add(new ClsAtoB() { iA = ia, iB = ib, });
            }

            System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
            sw.Start();

            // 解析
            long lOut = 0L;
            lstX.Sort(delegate (ClsAtoB mca1, ClsAtoB mca2) { return (int)(mca1.iA - mca2.iA); });       // iAの昇順
            long lNow = 0L;
            for (int i = 0; i < lstX.Count; i++)
            {
                lNow += lstX[i].iB;
                if (lNow >= iK) { lOut = lstX[i].iA; break; }
            }

            //出力
            Console.WriteLine("{0}", lOut.ToString().Trim());

            sw.Stop();
#if DEBUG
            Console.WriteLine();
            Console.WriteLine("{0}", sw.Elapsed.ToString());
            Console.WriteLine("続行するには何かキーを押してください．．．");
            Console.ReadKey();
#endif
    }
        private class ClsAtoB
        {
            public long iA { get; set; } = 0;
            public long iB { get; set; } = 0;
        }

}