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 (GCC 9.2.1) (judge verdict: Accepted)

#include<stdio.h>
//小さい順にソート
int c[100002][2];
void merge(int i,int j,int s[][2]){
  if(i!=j){
    merge(i,(i+j)/2,s);
    merge((i+j)/2+1,j,s);
    int a=i,b=(i+j)/2+1;
    int d;
    for(d=i;d<=j;d++){
      if((i+j)/2<a){
        c[d][0]=s[b][0];
        c[d][1]=s[b][1];
        b++;
      }
      else if(j<b){
        c[d][0]=s[a][0];
        c[d][1]=s[a][1];
        a++;
      }
      else if(s[a][0]<s[b][0]){
        c[d][0]=s[a][0];
        c[d][1]=s[a][1];
        a++;
      }
      else{
        c[d][0]=s[b][0];
        c[d][1]=s[b][1];
        b++;
      }
    }
    for(d=i;d<=j;d++){
      s[d][0]=c[d][0];
      s[d][1]=c[d][1];
    }
  }
}

int main(){
  int n,i;
  long sum=0,k;
  scanf("%d%ld",&n,&k);
  int a[n+1][2];
  for(i=0;i<n;i++)
    scanf("%d%d",&a[i][0],&a[i][1]);
  merge(0,n-1,a);
  i=0;
  while(sum+(long)a[i][1]<k){
    sum+=(long)a[i][1];
    i++;
  }
  printf("%d\n",a[i][0]);
  return 0;
}