Added functionality.

test.c

 #include <stdio.h>
+#include <stdlib.h>
 
 #define UNUSED(x) ((void)(x))
 
+int sum(int n)
+{
+    return ((n + 1) * n) / 2;
+}
+
+void usage(const char* app_name, const char* err_string)
+{
+    if(err_string != NULL) {
+        fprintf(stderr, "%s\n", err_string);
+    }
+    fprintf(stderr, "Usage: %s <n>\n", app_name);
+    exit(1);
+}
+
 int main(int argc, char* argv[])
 {
-    UNUSED(argc);
-    UNUSED(argv);
-    printf("Hallo Welt!\n");
+    if(argc != 2) {
+        usage(argv[0], NULL);
+        return 1;
+    }
+
+    int n = atoi(argv[1]);
+    if(n <= 0) {
+        usage(argv[1], "n must be a natural number.");
+    }
+
+    printf("Summe der nat. Zahlen von 1 - %i: %i\n", n, sum(n));
+
     return 0;
 }