print((function(f, s1, s2)
	return f(s1, s2)
end)(function (s1, s2)
	local k, n = s1:match("(%d+)%s(%d+)")
	local r = tonumber(k) * tonumber(n)
	s2:gsub("%d+", function (e) r = r - tonumber(e) end)
	return r < 0 and -1 or r
end, io.stdin:read("*l"), io.stdin:read("*l")))