Index: aoc2025.c
==================================================================
--- aoc2025.c
+++ aoc2025.c
@@ -13,10 +13,38 @@
 void aocYYYYDD(char *data, size_t len) {
     (void)len; // unused argument
 }
 
 #endif
+
+/* === aoc202509 =======================================================
+===================================================================== */
+struct iCoords {
+    int row, col;
+};
+
+void aoc202509(char *data, size_t len) {
+    (void)len; // unused argument
+    struct iCoords p[512] = {0};
+    char *err = data;
+    int k = 0;
+    while (*err) {
+        p[k].row = strtol(err, &err, 10);
+        if (*err != ',') break;
+        err++;
+        p[k++].col = strtol(err, &err, 10);
+    }
+    long long max_area = 0;
+    for (int i = 0; i < k; i++) {
+        for (int j = i + 1; j < k; j++) {
+            long long area = 1LL * (abs(p[i].col - p[j].col) + 1)
+                                 * (abs(p[i].row - p[j].row) + 1);
+            if (area > max_area) max_area = area;
+        }
+    }
+    printf("P1: %lld\n", max_area);
+}
 
 /* === aoc202508 =======================================================
 ===================================================================== */
 void aoc202508(char *data, size_t len) {
     (void)data; // unused argument

Index: aocdailies.c
==================================================================
--- aocdailies.c
+++ aocdailies.c
@@ -4,10 +4,11 @@
 aocfunc *aocselect(unsigned y, unsigned d) {
     aocfunc *p;
     switch (y * 100 + d) {
         default: p = NULL; break;
 
+        case 202509: p = aoc202509; break;
         case 202508: p = aoc202508; break;
         case 202507: p = aoc202507; break;
         case 202506: p = aoc202506; break;
         case 202505: p = aoc202505; break;
         case 202504: p = aoc202504; break;

Index: aocdailies.h
==================================================================
--- aocdailies.h
+++ aocdailies.h
@@ -4,10 +4,11 @@
 #include <stddef.h>
 
 typedef void aocfunc(char *, size_t);
 aocfunc *aocselect(unsigned, unsigned);
 
+aocfunc aoc202509;
 aocfunc aoc202508;
 aocfunc aoc202507;
 aocfunc aoc202506;
 aocfunc aoc202505;
 aocfunc aoc202504;