Problem Statement

$N students, numbered 1,2,\ldots,N,$ took an examination.
Student i$,(1 \leq i \leq N)$ had to score at least B_i points to graduate, where they actually scored A_i points.
You can repeat the following operation any number of times (possibly zero):

• Choose two students, and swap their scores.

Your goal is to make everyone graduate.
Determine whether it is possible.
If it is possible, find the maximum number of students whose scores do not change during the process.

Input

Input is given from Standard Input in the following format:
N
A_1 B_1
A_2 B_2
$\vdots$
A_N B_N

Output

If it is possible to make everyone graduate, print the maximum number of students whose scores do not change during the process.
Otherwise, print -1.

Constraints

• $2 \leq N \leq 3 \times 10^5$
• $1 \leq A_i,B_i \leq 10^9$$,(1 \leq i \leq N)$
• All values in input are integers.

Sample Input 1

3
1 2
3 1
3 3

Sample Output 1

1

If you swap scores of Student 1 and 2, everyone can graduate.
Here, the number of students whose scores do not change is 1 (only Student 3).

Sample Input 2

2
100 1
100 1

Sample Output 2

2

Sample Input 3

6
3 2
1 6
4 5
1 3
5 5
9 8

Sample Output 3

-1

Sample Input 4

6
3 1
4 5
5 2
2 3
5 4
5 1

Sample Output 4

3

见题面 Input 部分

见题面 Output 部分