diff --git a/README b/README
index 980a0d5..fd9fb4b 100644
--- a/README
+++ b/README
@@ -1 +1,4 @@
 Hello World!
+
+
+2026/08/14
diff --git a/main.cpp b/main.cpp
new file mode 100644
index 0000000..daf773e
--- /dev/null
+++ b/main.cpp
@@ -0,0 +1,18 @@
+#include <iostream>
+#include <vector>
+
+#include "segment_tree.hpp"
+
+int main() {
+    std::vector<long long> arr = {1, 3, 5, 7, 9, 11};
+
+    SumSegmentTree st = make_sum_segment_tree(arr);
+
+    std::cout << "Sum of [1, 3]: " << st.query(1, 3) << '\n';  // 3 + 5 + 7 = 15
+    std::cout << "Sum of [0, 5]: " << st.query(0, 5) << '\n';  // 36
+
+    st.update(2, 10);  // arr[2]: 5 -> 10
+    std::cout << "Sum of [1, 3] after update: " << st.query(1, 3) << '\n';  // 3 + 10 + 7 = 20
+
+    return 0;
+}
diff --git a/segment_tree.hpp b/segment_tree.hpp
new file mode 100644
index 0000000..df9b3e9
--- /dev/null
+++ b/segment_tree.hpp
@@ -0,0 +1,76 @@
+#pragma once
+
+#include <functional>
+#include <vector>
+
+template <typename T, typename Op>
+class SegmentTree {
+public:
+    SegmentTree(const std::vector<T>& data, Op op, T identity)
+        : n_(static_cast<int>(data.size())),
+          op_(std::move(op)),
+          identity_(identity),
+          tree_(4 * n_, identity_) {
+        if (n_ > 0) {
+            build(data, 0, 0, n_ - 1);
+        }
+    }
+
+    T query(int left, int right) const {
+        return query(0, 0, n_ - 1, left, right);
+    }
+
+    void update(int index, T value) {
+        update(0, 0, n_ - 1, index, value);
+    }
+
+private:
+    int n_;
+    Op op_;
+    T identity_;
+    std::vector<T> tree_;
+
+    void build(const std::vector<T>& data, int node, int start, int end) {
+        if (start == end) {
+            tree_[node] = data[start];
+            return;
+        }
+        int mid = start + (end - start) / 2;
+        build(data, 2 * node + 1, start, mid);
+        build(data, 2 * node + 2, mid + 1, end);
+        tree_[node] = op_(tree_[2 * node + 1], tree_[2 * node + 2]);
+    }
+
+    T query(int node, int start, int end, int left, int right) const {
+        if (right < start || end < left) {
+            return identity_;
+        }
+        if (left <= start && end <= right) {
+            return tree_[node];
+        }
+        int mid = start + (end - start) / 2;
+        return op_(
+            query(2 * node + 1, start, mid, left, right),
+            query(2 * node + 2, mid + 1, end, left, right));
+    }
+
+    void update(int node, int start, int end, int index, T value) {
+        if (start == end) {
+            tree_[node] = value;
+            return;
+        }
+        int mid = start + (end - start) / 2;
+        if (index <= mid) {
+            update(2 * node + 1, start, mid, index, value);
+        } else {
+            update(2 * node + 2, mid + 1, end, index, value);
+        }
+        tree_[node] = op_(tree_[2 * node + 1], tree_[2 * node + 2]);
+    }
+};
+
+using SumSegmentTree = SegmentTree<long long, std::plus<long long>>;
+
+inline SumSegmentTree make_sum_segment_tree(const std::vector<long long>& data) {
+    return SumSegmentTree(data, std::plus<long long>{}, 0LL);
+}
diff --git a/segment_tree_demo.exe b/segment_tree_demo.exe
new file mode 100644
index 0000000..acb3fe5
Binary files /dev/null and b/segment_tree_demo.exe differ