print((function (f, n)
	local r=f() if r<0 then return -1 end
	for i=2,n do r=f(r) if r<0 then return -1 end end
	return r
end)(function (n)
	local t = -io.stdin:read("*n")+io.stdin:read("*n")
	if n == nil then n = t end
	return n == t and n or -1
end, tonumber(io.stdin:read("*l"))))