This is known as the cancellation system. Pick a series of numbers, say 1,2,3,4,5. Your first bet is the total of the two numbers on the end, or 6 units in this case. If you lose it, you cancel out the 1 and the 5, add the six to the end of the series and bet the 6 and the 2, for a total of eight units bet. Lose that one and you add the eight to the series, cancel the 2 and the six and bet a total of 11 (3+8) and so on. If you win, you proceed to the inside numbers until they are all cancelled out. Again, a loser, but an insidious one.