def judge(n,fl)
    puts n if(fl == 1)
    e = 1
    while(e <= $a[0])
        if(e%2 == 1)
            n = ($a[e] - n)
            return 1 if(n < 0)
        else
            n -= $a[e] 
            return 2 if(n < 0)
        end
    e += 1
    puts n if(fl == 1)
    end
    return 0
end
$a = `dd`.split("\n").map{|u|u.to_i}
$za = 10**18
right = $a[1..3].max
left = 0
r = 0
while (right - left > 1)
    mid = left + (right + left)/2
    if(judge(mid, 0) == 1)
        left = mid
    elsif(judge(mid, 0) == 2)
        right = mid
    else
        puts $a[0] + 1
        judge(mid,1)
        exit
    end
    r+=1
    break if(r > 100000)
end
puts -1