diff --git a/ch4/helper.h b/ch4/helper.h
new file mode 100644
index 0000000..ca17d75
--- /dev/null
+++ b/ch4/helper.h
@@ -0,0 +1,3 @@
+
+
+extern int sum;
diff --git a/ch4/main.c b/ch4/main.c
new file mode 100644
index 0000000..3452ddc
--- /dev/null
+++ b/ch4/main.c
@@ -0,0 +1,19 @@
+#include <stdio.h>
+#include "helper.h"
+
+void increment();
+
+int main() {
+  printf("hello world\n");
+
+  increment();
+  increment();
+
+  printf("sum within main: %d\n", sum);
+
+  sum++;
+
+  printf("sum within main: %d\n", sum);
+
+  return 0;
+}
diff --git a/ch4/other.c b/ch4/other.c
new file mode 100644
index 0000000..a94971f
--- /dev/null
+++ b/ch4/other.c
@@ -0,0 +1,8 @@
+#include <stdio.h>
+#include "helper.h"
+
+int sum = 0;
+
+void increment() {
+  sum++;
+}