diff --git a/6.0s1.h b/6.0s1.h
index 67986e9..6d637c7 100644
--- a/6.0s1.h
+++ b/6.0s1.h
@@ -1,13 +1,15 @@
 #ifndef S1_6_0_H
 #define S1_6_0_H
 
+#include <inttypes.h>
+
 struct s1_location {
     const char *file;
     int line;
 };
 
-typedef int       s1_int;
-typedef long long s1_long;
+typedef int32_t   s1_int;
+typedef int64_t   s1_long;
 typedef double    s1_double;
 typedef char      s1_char;
 
@@ -37,12 +39,12 @@ void s1_trace(const char *text, struct s1_location loc) {
 // 2. simple IO
 
 void write_int(s1_int value, struct s1_location loc) {
-    int rc = printf("%d", value);
+    int rc = printf(PRId32, value);
     s1_assert(rc != EOF, "Error: write_int", loc);
 }
 
 void write_long(s1_long value, struct s1_location loc) {
-    int rc = printf("%lld", value);
+    int rc = printf(PRId64, value);
     s1_assert(rc != EOF, "Error: write_long", loc);
 }
 
@@ -63,14 +65,14 @@ void write_char(s1_char value, struct s1_location loc) {
 
 s1_int read_int(struct s1_location loc) {
     s1_int value = 0;
-    int rc = scanf("%d", &value);
+    int rc = scanf(SCNd32, &value);
     s1_assert(rc == 1, "Error: read_int", loc);
     return value;
 }
 
 s1_long read_long(struct s1_location loc) {
     s1_long value = 0;
-    int rc = scanf("%lld", &value);
+    int rc = scanf(SCNd64, &value);
     s1_assert(rc == 1, "Error: read_long", loc);
     return value;
 }