s, t, d = gets.split.map(&:to_i)
ws = gets.split.map(&:to_i)
min = ws.inject([0]) {|a, w| a + [a[-1] + w]}.min
week = ws.inject(:+)

if min >= 0
  puts -1
elsif week >= 0
  if s + min > t
    puts -1
  else
    (0..d-1).each do |i|
      s += ws[i]
      if s <= t
        puts i + 1
        break
      end
    end
  end
else
  k = 0
  until s + min <= t
    k += d
    s += week
  end
  (0..d-1).each do |i|
    s += ws[i]
    if s <= t
      puts k + i + 1
      break
    end
  end
end

