In Duolingo, it is the case that whenever the streak - number of days it is used in a row - reaches a number n divisible by 10, one obtains n/10 so called lingots. There is a possiblility to buy streak freezes, each costing 10 lingots. How many days in a row, starting from day zero, one needs to keep up the streak without using a streak freeze in order to gain so many lingots that one can only use streak freezes from that day on?