diff --git a/src/main/java/me/hatter/math/NormalDistribution.java b/src/main/java/me/hatter/math/NormalDistribution.java
index 41b08fc..25a5fea 100644
--- a/src/main/java/me/hatter/math/NormalDistribution.java
+++ b/src/main/java/me/hatter/math/NormalDistribution.java
@@ -30,16 +30,17 @@ public class NormalDistribution {
     }
 
     public static BigDecimal nd(BigDecimal mu, BigDecimal sigma, BigDecimal x) {
-        // \frac{1}{\sigma \times \sqrt{2\times \pi}}
+        // p_a=\frac{1}{\sigma \times \sqrt{2\times \pi}}
         BigDecimal pa = BigDecimal.ONE.divide(
                 sigma.multiply(MathUtil.sqrt(BigDecimal.valueOf(2).multiply(MathUtil.PI), 100))
                 , 100, RoundingMode.HALF_UP
         );
-        // \frac{(x-\mu)^2}{2\times \sigma^2}
+        // p_b=\frac{(x-\mu)^2}{2\times \sigma^2}
         BigDecimal pb = x.subtract(mu).pow(2).divide(
                 BigDecimal.valueOf(2).multiply(sigma.pow(2))
                 , 100, RoundingMode.HALF_UP
         );
+        // f(x)=p_a\times e^{-p_b}
         return pa.multiply(MathUtil.pow(MathUtil.E, BigDecimal.ZERO.subtract(pb)))
                 .setScale(100, RoundingMode.HALF_UP);
     }