diff --git a/AtCoder/ABC309/D/D.cpp b/AtCoder/ABC309/D/D.cpp
new file mode 100644
index 00000000..3d8baa82
--- /dev/null
+++ b/AtCoder/ABC309/D/D.cpp
@@ -0,0 +1,58 @@
+#include <iostream>
+#include <algorithm>
+#include <queue>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 3e5 + 5;
+
+int n1, n2, m, dist1[N], dist2[N];
+bool vis[N];
+std::vector<int> g[N];
+
+void bfs(int s, int* dist) {
+    std::queue<int> q;
+
+    q.emplace(s);
+    vis[s] = true;
+
+    while (!q.empty()) {
+        int u = q.front();
+        q.pop();
+
+        for (int v : g[u]) {
+            if (vis[v]) continue;
+
+            vis[v] = true;
+
+            dist[v] = dist[u] + 1;
+            q.emplace(v);
+        }
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n1 >> n2 >> m;
+
+    for (int i = 1, u, v; i <= m; i++) {
+        cin >> u >> v;
+
+        g[u].emplace_back(v);
+        g[v].emplace_back(u);
+    }
+
+    bfs(1, dist1);
+    bfs(n1 + n2, dist2);
+
+    cout << *std::max_element(dist1 + 1, dist1 + 1 + n1)
+                + *std::max_element(dist2 + n1 + 1, dist2 + n1 + n2 + 1) + 1
+         << endl;
+
+    return 0;
+}