We consider a branching-selection particle system on the real line. In this model the total size of the population at time n is limited by exp an 1/3. At each step n, every individual dies while reproducing independently, making children around their current position according to i.i.d. point processes. Only the exp a(n + 1) 1/3 rightmost children survive to form the (n + 1)th generation. This process can be seen as a generalisation of the branching random walk with selection of the N rightmost individuals, introduced by Brunet and Derrida in [9]. We obtain the asymptotic behaviour of position of the extremal particles alive at time n by coupling this process with a branching random walk with a killing boundary.