physiology - How does extracellular potassium ion concentration and calcium ion concentration affect the excitability of a cell?

When extracellular $K^+$ concentration increase by a certain amount, excitability of cells is higher because the resting potential shifts toward a higher equilibrium potential of $K^+$, therefore causing depolarization. However, when there concentration of extracellular $Ca^{2+}$ increases, excitability of cells decreases. If $Ca^{2+}$ and $K^+$ are both positively charged ions, what is the cause behind their opposite physiological effects?

I'm not a biology student, so detailed answers would be kindly appreciated. Thank you!