import math
def main():
    n, k = map(int, input().split())
    ab = [list(map(int, input().split())) for i in range(n)]

    def ok(v):

        s = 0
        for i in range(n):
            # 境界より大きい硬貨はカウントしない
            s += max(0, min(v, ab[i][1]) - ab[i][0] + 1)

        if s >= k:
            return True
        else:
            return False

    l, r = 0, 10**18
    for i in range(200):
        m = (l + r) / 2
        if ok(m):
            r = m
        else:
            l = m
    print(math.ceil(m))


if __name__ == '__main__':
    main()