make sure the factor is valid (also accept non-base 10 numbers)

dpu.c

@@ -1,3 +1,4 @@
+#include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -7,7 +8,13 @@ int main (int argc, char **argv){
 	FILE *bob,*joe,*moe;
 	int thing, factor, arg;
 	if(argc >= 3){
-		factor = atoi (argv[1]);
+        char *end_char;
+		factor = strtol(argv[1], &end_char, 0);
+        if(!*(argv[1]) || *end_char)
+        {
+            fputs("I can haz number?\n", stderr);
+            return EXIT_FAILURE;
+        }
 		arg = 2;
 	}else{
 		factor = 9;