diff --git a/combinatorial_opt/linear_sum_assignment.md b/combinatorial_opt/linear_sum_assignment.md
index 19721510..6e8ec759 100644
--- a/combinatorial_opt/linear_sum_assignment.md
+++ b/combinatorial_opt/linear_sum_assignment.md
@@ -60,6 +60,7 @@ for (int t = 0; t < k; ++t) {
 ## 問題例
 
 - [Library Checker: Assignment Problem](https://judge.yosupo.jp/problem/assignment)
+- [No.3306 Life is Easy? - yukicoder](https://yukicoder.me/problems/no/3306)
 
 ## 文献・リンク集
 
diff --git a/combinatorial_opt/test/linear_sum_assignment.yuki3306.test.cpp b/combinatorial_opt/test/linear_sum_assignment.yuki3306.test.cpp
new file mode 100644
index 00000000..f15dfc87
--- /dev/null
+++ b/combinatorial_opt/test/linear_sum_assignment.yuki3306.test.cpp
@@ -0,0 +1,31 @@
+#define PROBLEM "https://yukicoder.me/problems/no/3306"
+
+#include "../linear_sum_assignment.hpp"
+
+#include <iostream>
+#include <vector>
+using namespace std;
+
+int main() {
+    cin.tie(nullptr), ios::sync_with_stdio(false);
+
+    int N, M;
+    cin >> N >> M;
+    vector A(N, vector<int>(M));
+    for (auto &v : A) {
+        for (auto &x : v) cin >> x;
+    }
+
+    const int K = N / 2;
+    vector mat(K, vector<long long>(K));
+    for (int i = 0; i < K; ++i) {
+        for (int j = 0; j < K; ++j) {
+            int best = 0;
+            for (int k = 0; k < M; ++k) best = max(best, A.at(N - 1 - j).at(k) - A.at(i).at(k));
+            mat.at(i).at(j) = -best;
+        }
+    }
+
+    auto res = linear_sum_assignment::solve(K, K, mat);
+    cout << -res.opt << '\n';
+}
diff --git a/utilities/floor_sum.hpp b/utilities/floor_sum.hpp
index ada8ab05..c37ef0c5 100644
--- a/utilities/floor_sum.hpp
+++ b/utilities/floor_sum.hpp
@@ -6,6 +6,7 @@
 // 1 <= m < 2e32 (if Int is long long)
 // 0 <= a, b < m
 // Complexity: O(lg(m))
+// (Int, Unsigned) = (long long, unsigned long long), (__int128_t, __uint128_t)
 template <class Int, class Unsigned> Int floor_sum(Int n, Int m, Int a, Int b) {
     static_assert(-Int(1) < 0, "Int must be signed");
     static_assert(-Unsigned(1) > 0, "Unsigned must be unsigned");
diff --git a/utilities/test/floor_sum.int128.yuki3307.test.cpp b/utilities/test/floor_sum.int128.yuki3307.test.cpp
new file mode 100644
index 00000000..37832515
--- /dev/null
+++ b/utilities/test/floor_sum.int128.yuki3307.test.cpp
@@ -0,0 +1,32 @@
+#define PROBLEM "https://yukicoder.me/problems/no/3307"
+#include "../floor_sum.hpp"
+#include <iostream>
+using namespace std;
+
+int main() {
+    cin.tie(nullptr), ios::sync_with_stdio(false);
+
+    long long A, B, C, D;
+    cin >> A >> B >> C >> D;
+
+    if (A * D == B * C) {
+        cout << "-1\n";
+        return 0;
+    }
+
+    if (A * D > C * B) swap(A, C), swap(B, D);
+
+    // round(i * A / B) = floor(iA/B + 1/2) = floor((2iA + B)/2B)
+
+    // A/B < C/D
+    // i(A/B) + 1 <= i(C/D)
+    // i(C/D - A/B) >= 1
+    // i(BC - AD) >= BD
+
+    const auto det = B * C - A * D;
+    const auto ub = (B * D + det - 1) / det;
+
+    const auto ret = floor_sum<__int128_t, __uint128_t>(ub, D * 2, C * 2, D) -
+                     floor_sum<__int128_t, __uint128_t>(ub, B * 2, A * 2, B);
+    cout << (long long)(ub - ret - 1) << '\n';
+}