The dissociation constant for a receptor-ligand pair is \( 0.25 \times 10^{-7} \, \text{M} \). The ligand was added to a solution of the receptor such that the receptor was 50% saturated at equilibrium. Assume that the receptor has one ligand binding site. The concentration of the free ligand at equilibrium in nM, correct to the nearest integer, should be ________.