Calcium Folinate Impurity: (2S)-2-(4-(((2-Amino-5-((4S)-4-(4-(((2-amino-5-formyl-4-oxo-1,4,5,6,7,8-hexahydropteridin-6-yl)methyl)amino)benzamido)-4-carboxybutanoyl)-4-oxo-1,4,5,6,7,8-hexahydropteridin-6-yl)methyl)amino)benzamido)pentanedioic Acid [5-.