diff --git a/flydongwoo/AlgoLeadMe_Week21_prob01.cpp b/flydongwoo/AlgoLeadMe_Week21_prob01.cpp
new file mode 100644
index 0000000..a9074c5
--- /dev/null
+++ b/flydongwoo/AlgoLeadMe_Week21_prob01.cpp
@@ -0,0 +1,75 @@
+#include <iostream>
+#include <queue>
+#include <cstring>
+
+#define MAX_SIZE 1000 + 1
+
+using namespace std;
+
+struct tomato {
+    int y, x;
+};
+
+queue<tomato> q;
+
+// ¿ì,ÇÏ,ÁÂ,»ó
+int dx[4] = { 1, 0, -1, 0 };
+int dy[4] = { 0, 1, 0, -1 };
+
+int n, m, result = 0;
+int graph[MAX_SIZE][MAX_SIZE];
+
+bool IsInside(int ny, int nx) {
+    return (0 <= nx && 0 <= ny && nx < m && ny < n);
+}
+
+void bfs(void) {
+    while (!q.empty()) {
+        int y = q.front().y;
+        int x = q.front().x;
+        q.pop();
+
+        for (int i = 0; i < 4; i++) {
+            int ny = y + dy[i];
+            int nx = x + dx[i];
+
+            if (IsInside(ny, nx) == 1 && graph[ny][nx] == 0) {
+                graph[ny][nx] = graph[y][x] + 1;
+                q.push({ ny, nx });
+            }
+        }
+    }
+}
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> m >> n;
+
+    for (int i = 0; i < n; i++) {
+        for (int j = 0; j < m; j++) {
+            cin >> graph[i][j];
+            if (graph[i][j] == 1) {
+                q.push({ i, j });
+            }
+        }
+    }
+
+    bfs();
+
+    for (int i = 0; i < n; i++) {
+        for (int j = 0; j < m; j++) {
+            if (graph[i][j] == 0) {
+                cout << -1 << "\n";
+                return 0;
+            }
+            if (result < graph[i][j]) {
+                result = graph[i][j];
+            }
+        }
+    }
+
+    cout << (result - 1) << endl;
+    return 0;
+}
diff --git a/flydongwoo/README.md b/flydongwoo/README.md
index 3e5cafc..3c025a9 100644
--- a/flydongwoo/README.md
+++ b/flydongwoo/README.md
@@ -21,5 +21,6 @@
 | 17차시 | 2025.08.06 |  DFS와 BFS  | [바�러스](https://www.acmicpc.net/problem/2606)|https://github.com/AlgoLeadMe/AlgoLeadMe-15/pull/63|
 | 18차시 | 2025.08.07 |  BFS  | [숨바꼭질](https://www.acmicpc.net/problem/1697)|https://github.com/AlgoLeadMe/AlgoLeadMe-15/pull/64|
 | 19차시 | 2025.08.14 |  Sliding Window  | [수열](https://www.acmicpc.net/problem/2559)|https://github.com/AlgoLeadMe/AlgoLeadMe-15/pull/71|
-| 20차시 | 2025.08.27 |  Greedy Algorithm  | [ATM](https://www.acmicpc.net/problem/2559)|https://github.com/AlgoLeadMe/AlgoLeadMe-15/pull/81|
+| 20차시 | 2025.08.27 |  Greedy Algorithm  | [ATM](https://www.acmicpc.net/problem/11399)|https://github.com/AlgoLeadMe/AlgoLeadMe-15/pull/81|
+| 21차시 | 2025.08.28 |  BFS  | [토마토](https://www.acmicpc.net/problem/7576)|https://github.com/AlgoLeadMe/AlgoLeadMe-15/pull/82|