diff --git a/number/examples/factorial-inverse-in-linear.cc b/number/examples/factorial-inverse-in-linear.cc
index 55baa7d..cf49f5f 100644
--- a/number/examples/factorial-inverse-in-linear.cc
+++ b/number/examples/factorial-inverse-in-linear.cc
@@ -1,20 +1,27 @@
+constexpr int M = MDL;
+
 constexpr int N = 3e6 + 10;
 ll fact[N], factrev[N + 1], s[N + 1];
 
 void prep() {
     fact[0] = factrev[0] = 1;
     for (int i = 1; i < N; ++i) {
-        fact[i] = (fact[i - 1] * i) % MDL;
+        fact[i] = (fact[i - 1] * i) % M;
     }
     s[0] = 1;
     for (int i = 1; i <= N; ++i) {
-        s[i] = s[i - 1] * fact[i - 1] % MDL;
+        s[i] = s[i - 1] * fact[i - 1] % M;
     }
-    factrev[N] = inverse(s[N], MDL);
+    factrev[N] = inverse(s[N], M);
     for (int i = N; i; --i) {
-        factrev[i - 1] = factrev[i] * fact[i - 1] % MDL;
+        factrev[i - 1] = factrev[i] * fact[i - 1] % M;
     }
     for (int i = 0; i < N; ++i) {
-        factrev[i] = factrev[i + 1] * s[i] % MDL;
+        factrev[i] = factrev[i + 1] * s[i] % M;
     }
 }
+
+ll mycomb(int n, int k) {
+    if (n < 0 or k < 0 or n < k) return 0;
+    return fact[n] * factrev[k] % M * factrev[n - k] % M;
+}