def judge(x):
    cnt = 0
    
    for Ai, Bi in AB:
        if Ai <= x <= Bi:
            cnt += x - Ai + 1
        elif Bi < x:
            cnt += Bi - Ai + 1
    
    if cnt >= K:
        return True
    else:
        return False
    
def binary_search():
    left, right = 0, 10 ** 18
    
    while left <= right:
        mid = (left + right) // 2
        
        if judge(mid):
            right = mid - 1
        else:
            left = mid + 1
    
    return left

N, K = map(int, input().split())
AB = [tuple(map(int, input().split())) for _ in range(N)]
ans = binary_search()
print(ans)