문제
한 농부가 할로윈 파티에 그의 소들을 데려가려고한다. 아쉽게도 농부에게는 코스튬이 한벌밖에 없다. 그 코스튬에는 정확하게 사이즈는 S(1 <= S <= 1,000,000)이며, 최대 소 두마리가 들어간다. 농부는 N(2 <= N <= 20,000)마리의 소가 있으며(소의 이름은 편의상 소1.. 소N으로한다), 소i의 사이즈는 (1 <= L_i <= 1,000,000)이다. 만약 소 두마리의 크기 합이 코스튬의 크기 이하인 경우 둘이 코스튬에 들어갈 수 있다. 농부가 코스튬에 얼마나 많은 서로 다른 소의 짝이 들어가는지 구할수있도록 도와주자.
출력
첫째 줄에 얼마나 많은 짝이 가능한지 출력한다.
힌트
위의 문제에서 4개의 짝은 아래와 같다.
- 소1-소3 (사이즈 합:5)
- 소1-소4 (사이즈 합:4)
- 소2-소4 (사이즈 합:6)
- 소3-소4 (사이즈 합:3)
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