#!/usr/bin/env ruby

input = $stdin.readlines.map(&:chomp).map(&:to_i)
pc = 0
steps = 0

while pc >= 0 && pc < input.length do
  newpc = pc + input[pc]
  input[pc] += input[pc] >= 3 ? -1 : 1
  pc = newpc
  steps += 1
end

puts steps