# Here your code !
INF = 10 ** 7
c,n = 2.times.map { gets.to_i }
a = gets.split.map(&:to_i).sort
dp = Array.new(c+1,INF)
dp[0] = 0
(0..c).each do |i|
    if dp[i] < INF
        n.times do |j|
            if i + a[j] <= c
                dp[i + a[j]] = [dp[i + a[j]], dp[i] + 1].min
            end
        end
    end
end
dp[c] = -1 if dp[c] > c
puts dp[c]