A quantitative analyst at a proprietary trading firm is incorporating unsupervised machine learning (ML) algorithms into the firm's technical analysis of equities by using K-means clustering. The clustering algorithm will be applied to continuous volatility data in order to group observations into clusters that can be used to identify the current market regime. Since clusters close to each other are likely to exhibit similar characteristics, the analyst measures the distances between the observations within each cluster and the centroid of that cluster. If the analyst wants to ensure that the minimum distance is obtained for the continuous data clusters when applying the K-means algorithm, which measure should be used to achieve the desired outcome?