문제
상근이는 등차수열을 이루는 정수 4개를 골랐다. 이것은 상근이가 고른 수 4개를 정렬했을 때, 인접한 쌍의 차이가 일정하다는 것을 의미한다. 그 다음 이렇게 고른 수 4개를 노래로 만들어서 외우고 다닌다.
어느 날, 상근이는 자신이 고른 4개 수 중 1개를 까먹었다. 상근이가 기억하고 있는 수 세 개가 주어졌을 때, 까먹은 수를 구하는 프로그램을 작성하시오.
출력
첫째 줄에 까먹은 수를 출력한다. 정답이 여러 개일 경우에는 아무거나 출력하면 된다. 또한 정답이 존재하지 않는 경우는 입력으로 주어지지 않는다.
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