The approximate Shortest Vector Problem (apprSVP) is a problem where, given the basis and the approximation factor $\gamma$ (a function of the dimension $n$), one must find a vector $v$ belonging to the lattice $L$ such that its norm is less than $\gamma$ times the length of the shortest vector in the lattice $L$.
But it is hard to find even the length of the shortest vector, so then what value should we substitute in place of the length of the shortest vector when defining apprSVP?