diff --git a/src/main/java/me/hatter/math/Crt.java b/src/main/java/me/hatter/math/Crt.java
new file mode 100644
index 0000000..1df022d
--- /dev/null
+++ b/src/main/java/me/hatter/math/Crt.java
@@ -0,0 +1,40 @@
+package me.hatter.math;
+
+import java.math.BigInteger;
+
+public class Crt {
+
+    public static void main(String[] args) {
+        System.out.println(crt(
+                new BigInteger[]{
+                        BigInteger.valueOf(2),
+                        BigInteger.valueOf(3),
+                        BigInteger.valueOf(2),
+                },
+                new BigInteger[]{
+                        BigInteger.valueOf(3),
+                        BigInteger.valueOf(5),
+                        BigInteger.valueOf(7),
+                }
+        ));
+    }
+
+    // https://oi-wiki.org/math/number-theory/crt/
+    public static BigInteger crt(BigInteger[] as, BigInteger[] ns) {
+        BigInteger n = BigInteger.ONE;
+        // \prod n_i
+        for (BigInteger ni : ns) {
+            n = n.multiply(ni);
+        }
+        BigInteger x = BigInteger.ZERO;
+        for (int i = 0; i < as.length; i++) {
+            // m_i = n / n_i
+            BigInteger mi = n.divide(ns[i]);
+            // c_i = m_i^-1 mod n_i
+            BigInteger miInv = mi.modInverse(ns[i]);
+            // \sum a_i m_i c_i mod n
+            x = x.add(as[i].multiply(mi.multiply(miInv))).mod(n);
+        }
+        return x.mod(n).add(n).mod(n);
+    }
+}