Ivan has got an array of n non-negative integers a1, a2, ..., an. Ivan knows that the array is sorted in the non-decreasing order. 

Ivan wrote out integers 2a1, 2a2, ..., 2an on a piece of paper. Now he wonders, what minimum number of integers of form 2b (b ≥ 0) need to be added to the piece of paper so that the sum of all integers written on the paper equalled 2v - 1 for some integer v (v ≥ 0). 

Help Ivan, find the required quantity of numbers.

Input

The first line contains integer n (1 ≤ n ≤ 105). The second input line contains n space-separated integers a1, a2, ..., an (0 ≤ ai ≤ 2·109). It is guaranteed that a1 ≤ a2 ≤ ... ≤ an.

Output

Print a single integer — the answer to the problem.

Examples

Input

4
0 1 1 1


Output

0


Input

1
3


Output

3

Note

In the first sample you do not need to add anything, the sum of numbers already equals 23 - 1 = 7.

In the second sample you need to add numbers 20, 21, 22.
import io
import os
import sys
from atexit import register
import random
import math
import itertools

##################################### Flags          #####################################
DEBUG = True
STRESSTEST = False

##################################### IO             #####################################

if not DEBUG:
    input = io.BytesIO(os.read(0, os.fstat(0).st_size)).readline
sys.stdout = io.BytesIO()
register(lambda: os.write(1, sys.stdout.getvalue()))

tokens = []
tokens_next = 0


def nextStr():
    global tokens, tokens_next
    while tokens_next >= len(tokens):
        tokens = input().split()
        tokens_next = 0
    tokens_next += 1

    if type(tokens[tokens_next - 1]) == str:
        return tokens[tokens_next - 1]
    return tokens[tokens_next - 1].decode()


def nextInt():
    return int(nextStr())


def nextIntArr(n):
    return [nextInt() for i in range(n)]


def print(*argv, end='\n'):
    for arg in argv:
        sys.stdout.write((str(arg) + ' ').encode())
    sys.stdout.write(end.encode())


##################################### Helper Methods #####################################
def genTestCase():
    raise NotImplementedError


def bruteforce(a):
    raise NotImplementedError


def doStressTest():
    while True:
        curTest = genTestCase()
        mySoln = solve(curTest)
        bruteforceSoln = bruteforce(curTest)
        if mySoln != bruteforceSoln:
            print('### Found case ###')
            print(curTest)
            print(f'{mySoln} should have been: {bruteforceSoln}')
            return


def solve(a):
    s = set()
    for c in a:
        if c in s:
            t = c
            while t in s:
                s.remove(t)
                t += 1
            s.add(t)
        else:
            s.add(c)
    return max(s) - len(s) + 1


##################################### Driver         #####################################

if __name__ == "__main__":
    if not DEBUG and STRESSTEST:
        raise Exception('Wrong flags!')

    if STRESSTEST:
        doStressTest()
    else:
        ### Read input here
        a = nextIntArr(nextInt())
        res = solve(a)
        print(res)

    sys.stdout.flush()