From 6e2b2587a10da903d588318faa28f9af66ba8c50 Mon Sep 17 00:00:00 2001
From: rsk0315 <rsk0315@icloud.com>
Date: Wed, 16 Dec 2020 14:41:15 +0900
Subject: [PATCH 1/3] Improve floor_sum

---
 atcoder/math.hpp | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/atcoder/math.hpp b/atcoder/math.hpp
index ec5bca5..13fbf6b 100644
--- a/atcoder/math.hpp
+++ b/atcoder/math.hpp
@@ -90,10 +90,11 @@ long long floor_sum(long long n, long long m, long long a, long long b) {
         b %= m;
     }
 
-    long long y_max = (a * n + b) / m, x_max = (y_max * m - b);
-    if (y_max == 0) return ans;
-    ans += (n - (x_max + a - 1) / a) * y_max;
-    ans += floor_sum(y_max, a, m, (a - x_max % a) % a);
+    long long y_max = a * (n - 1) + b;
+    if (y_max < m) return ans;
+    long long y_max_div = y_max / m;
+    long long y_max_mod = y_max % m;
+    ans += y_max_div + floor_sum(y_max_div, a, m, y_max_mod);
     return ans;
 }
 

From 42011962c40ad184210c57511b58faede18bff14 Mon Sep 17 00:00:00 2001
From: rsk0315 <rsk0315@icloud.com>
Date: Wed, 16 Dec 2020 15:46:48 +0900
Subject: [PATCH 2/3] Change the order of operation in case of the overflow

---
 atcoder/math.hpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/atcoder/math.hpp b/atcoder/math.hpp
index 13fbf6b..f8723a4 100644
--- a/atcoder/math.hpp
+++ b/atcoder/math.hpp
@@ -82,7 +82,7 @@ std::pair<long long, long long> crt(const std::vector<long long>& r,
 long long floor_sum(long long n, long long m, long long a, long long b) {
     long long ans = 0;
     if (a >= m) {
-        ans += (n - 1) * n * (a / m) / 2;
+        ans += (n - 1) * n / 2 * (a / m);
         a %= m;
     }
     if (b >= m) {

From a596e83d4ca193eb220dee00ccc908373a6a3117 Mon Sep 17 00:00:00 2001
From: rsk0315 <rsk0315@icloud.com>
Date: Wed, 16 Dec 2020 15:49:16 +0900
Subject: [PATCH 3/3] Reduce operations per function call

---
 atcoder/math.hpp | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/atcoder/math.hpp b/atcoder/math.hpp
index f8723a4..0b6e366 100644
--- a/atcoder/math.hpp
+++ b/atcoder/math.hpp
@@ -90,11 +90,9 @@ long long floor_sum(long long n, long long m, long long a, long long b) {
         b %= m;
     }
 
-    long long y_max = a * (n - 1) + b;
+    long long y_max = a * n + b;
     if (y_max < m) return ans;
-    long long y_max_div = y_max / m;
-    long long y_max_mod = y_max % m;
-    ans += y_max_div + floor_sum(y_max_div, a, m, y_max_mod);
+    ans += floor_sum(y_max / m, a, m, y_max % m);
     return ans;
 }