#!/usr/bin/env python3

import sys

m = int(sys.stdin.readline())

vs = {}
maxv, minv = 0, 5000
for _ in range(m):
  v = int(sys.stdin.readline()) + 1
  vs[v] = vs.get(v, 0) + 1
  maxv = max(maxv, v)
  minv = min(minv, v)

for n in range(1, maxv+1):
  accommodates = [0] + list(range(1,n+1)) + list(range(n-1,0,-1))
  ok = True
  for v in range(minv,maxv+1):
    if v >= len(accommodates) or vs.get(v, 0) > accommodates[v]:
      ok = False
      break
  if ok:
    print(n)
    sys.exit(0)