diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..8326e08
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+*.o
+build/
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..6ad58cb
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,26 @@
+
+CC 		 = gcc
+CFLAGS 	 = -Wall -pedantic -ansi
+LDFLAGS  = -lm -lgmp
+BUILDDIR = build
+LIBDIR 	 = lib
+
+STRIP	 = strip
+RMDIR	 = rm -fr
+RM 		 = rm -f
+
+p% : p%.c 
+	mkdir -p $(BUILDDIR)
+	$(CC) $(CFLAGS) $(LDFLAGS) $^ -o $(BUILDDIR)/$@
+	$(STRIP) $(BUILDDIR)/$@
+
+p7 : p7.c $(LIBDIR)/prime.o
+p10 : p10.c $(LIBDIR)/prime.o
+p12 : p12.c $(LIBDIR)/prime.o
+
+$(LIBDIR)/%.o : $(LIBDIR)/%.c
+	$(CC) $(CFLAGS) -c $< -o $@
+
+clean :
+	$(RMDIR) $(BUILDDIR)
+	$(RM) $(LIBDIR)/*.o
diff --git a/p10.c b/p10.c
index e2ee6fd..a853fe6 100644
--- a/p10.c
+++ b/p10.c
@@ -19,7 +19,7 @@ int main() {
         if (is_prime(i))
             s += i;
     }
-    printf("%lli\n", s); /* this format throws som warnings but works so whatever */
+    printf("%li\n", s);
 
     return 0;
 }
diff --git a/p11.c b/p11.c
index 9f27085..6f9d49f 100644
--- a/p11.c
+++ b/p11.c
@@ -20,6 +20,7 @@
 /* macro for checking all directions of a square x,y */
 #define check_square(x, y)      \
     do {                        \
+        int p;                  \
         if (x < 17)             \
             setprod(px(x, y));  \
         if (y < 17)             \
@@ -30,6 +31,13 @@
             setprod(prd(x, y)); \
     } while(0)
 
+#define setprod(x)      \
+    do {                \
+        p = x;          \
+        if (p > result) \
+            result = p; \
+    } while(0)
+
 int grid[20][20] = {
     {  8,  2, 22, 97, 38, 15,  0, 40,  0, 75,  4,  5,  7, 78, 52, 12, 50, 77, 91,  8 },
     { 49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48,  4, 56, 62,  0 },
@@ -55,12 +63,6 @@ int grid[20][20] = {
 
 unsigned int result = 0;
 
-void inline setprod(int p) {
-
-    if (p > result)
-        result = p;
-}
-
 int main() {
 
     int x, y;
diff --git a/p2.c b/p2.c
index 878e86e..f3bd99d 100644
--- a/p2.c
+++ b/p2.c
@@ -11,16 +11,18 @@
 
 int main() {
 	
-	int c = 0, a = 1, b = 1, r = 0;
+	int a = 1, b = 1, r = 0;
 		
-	while(c < 4000000) {
+	for(;;) {
 		
-		c = a + b;
+		int c = a + b;
 		a = b;
 		b = c;
-			
-		if (!(c & 1))
+
+		if (~c & 1)
 			r += c;
+        if (c >= 4000000)
+            break;
 	}
 		
 	printf("%i\n", r);
diff --git a/p4.c b/p4.c
index 19fbc09..62a4d9f 100644
--- a/p4.c
+++ b/p4.c
@@ -50,7 +50,7 @@ int main() {
 		if (!ispalindrom(r) || !(a = factor(r)))
 			continue;
 		
-		printf("%i * %i = %i\n", r/a, a, r);
+		printf("%i\n", r);
 		break;
 	}
 	
diff --git a/p6.c b/p6.c
index 8a9532e..8c8120f 100644
--- a/p6.c
+++ b/p6.c
@@ -19,4 +19,6 @@ int main() {
     }
     
     printf("%i\n", s2*s2 - s1);
+
+    return 0;
 }