local mfl, mce = math.floor, math.ceil
local n = io.read("*n")
local a = {}
for i = 1, n do
  a[i] = io.read("*n")
end
local xor = 0
for i = 3, n do
  xor = xor ~ a[i]
end
local sum = a[1] + a[2]
if sum < xor then
  print(-1)
  os.exit()
end
local b_xor, b_sum = {}, {}
while 0 < sum do
  table.insert(b_sum, sum % 2)
  table.insert(b_xor, xor % 2)
  sum = mfl(sum / 2)
  xor = mfl(xor / 2)
end
local dig = #b_sum
local moveup = false
local state = {}
for i = 1, dig do state[i] = 0 end
-- state of bit of a[1] and a[2]
-- 1: [0, 0], 2: [1, 1], 3: [0, 1] or [1, 0]
for idig = dig, 1, -1 do
  if b_sum[idig] == 1 then
    if b_xor[idig] == 1 then
      if moveup then
        print(-1)
        os.exit()
      else
        state[idig] = 3
      end
    else
      if moveup then
        state[idig] = 2
      else
        state[idig] = 1
      end
      moveup = true
    end
  else-- sum = 0
    if b_xor[idig] == 1 then
      if moveup then
        state[idig] = 3
      else
        print(-1)
        os.exit()
      end
    else
      if moveup then
        state[idig] = 2
      else
        state[idig] = 1
      end
      moveup = false
    end
  end
end
if moveup then print(-1) os.exit() end
local alim = {}
do
  local tmp = a[1]
  for i = 1, dig do
    alim[i] = tmp % 2
    tmp = mfl(tmp / 2)
  end
end
local tbl = {}
for i = 1, dig do tbl[i] = 0 end

local function dfs(idig, limit)
  if idig == 0 then return true end
  if limit then
    if alim[idig] == 0 then
      if state[idig] == 2 then
        return false
      else
        tbl[idig] = 0
        return dfs(idig - 1, true)
      end
    else
      if state[idig] == 2 then
        tbl[idig] = 1
        return dfs(idig - 1, true)
      elseif state[idig] == 1 then
        tbl[idig] = 0
        return dfs(idig - 1, false)
      else
        tbl[idig] = 1
        local ret = dfs(idig - 1, true)
        if ret then
          return true
        else
          tbl[idig] = 0
          return dfs(idig - 1, false)
        end
      end
    end
  else
    if state[idig] == 1 then
      tbl[idig] = 0
      return dfs(idig - 1, false)
    else
      tbl[idig] = 1
      return dfs(idig - 1, false)
    end
  end
end

local ret = dfs(dig, true)
if not ret then
  print(-1)
  os.exit()
end
local ans = 0
local mul = 1
for i = 1, dig do
  ans = ans + mul * tbl[i]
  mul = mul * 2
end
if ans == 0 then
  print(-1)
else
  print(a[1] - ans)
end
