diff --git a/.gitignore b/.gitignore
index b65d4ef..5b0dbe3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -50,3 +50,5 @@ day4/U287193/chat
 day4/U461920/fixed
 day5/hard/hard
 day5/perfect/perfect
+day5/good/good
+day5/fair/fair
diff --git a/README.md b/README.md
index 9333274..88ada7b 100644
--- a/README.md
+++ b/README.md
@@ -44,7 +44,27 @@ int main(){
 
 ## Day5
 ### 需要学习的点
->区间最值
+#### 区间最值
+#### 弄懂分数取模出现很大的数字是什么鬼、
+1. >费马小定理
+2. >逆元
+```cpp
+// a/b%M
+ll _ksm(ll a, ll b,ll M){
+    ll res = 1;
+    while(b) {
+        if(b & 1)                    
+            res = res * a % M;       
+        a = a * a % M;               
+        b >>= 1;                     
+    }
+    return res;
+}
+
+ll ksm(ll a,ll b,ll M){
+    return a * _ksm(b, M - 2,M) % M;
+}
+```
 
 # 排序
 ## 稳定性
diff --git a/day5/fair/fair.cpp b/day5/fair/fair.cpp
new file mode 100644
index 0000000..ade2c41
--- /dev/null
+++ b/day5/fair/fair.cpp
@@ -0,0 +1,55 @@
+#include<bits/stdc++.h>
+using namespace std;
+
+#ifdef DEBUG
+#define PRINT_VALUE(v){cout<<#v<<" :"<<(v)<<endl;}
+#endif
+
+#ifndef DEBUG
+#define PRINT_VALUE(v)
+#endif
+
+#define ll long long
+#define int long long
+
+ll _ksm(ll a, ll b,ll M){
+    ll res = 1;
+    while(b) {
+        if(b & 1)                    
+            res = res * a % M;       
+        a = a * a % M;               
+        b >>= 1;                     
+    }
+    return res;
+}
+
+ll ksm(ll a,ll b,ll M){
+    return a * _ksm(b, M - 2,M) % M;
+}
+
+int n,k,m,times=1,b;
+
+signed main(){
+    cin>>n>>k>>m;
+    b=k;
+    /*
+    x=k+x/b
+    bx=k*b+x
+    (b-1)x=k*b
+    x=(k*b)/(b-1)
+    */
+    PRINT_VALUE(b);
+    PRINT_VALUE(n);
+    PRINT_VALUE(b<n);
+    while(b<n){
+        PRINT_VALUE(b<n);
+        PRINT_VALUE(b*k);
+        b=b*k;
+        times++;
+        PRINT_VALUE(times);
+    }
+    PRINT_VALUE(k);
+    PRINT_VALUE(b);
+    PRINT_VALUE(ksm(k*b,b-1,m));
+    cout<<ksm(k*b,b-1,m)<<endl;
+}
\ No newline at end of file