diff --git a/main.c b/main.c
index f7a2efd..c0d02b6 100644
--- a/main.c
+++ b/main.c
@@ -65,7 +65,7 @@ int main(void)
 		putchar('\b');
 	puts("100%");
 
-	bool res[NPUZZLES];
+	int res[NPUZZLES];
 	struct timespec start, end;
 	fputs("Solving... ", stdout);
 	fflush(stdout);
@@ -85,14 +85,21 @@ int main(void)
 	const double avg_micros = tot_micros / NPUZZLES;
 
 	unsigned solved = 0;
+	unsigned tot_passes = 0;
 	for (i = 0; i < NPUZZLES; ++i) {
-		if (res[i] && check(&puzzles[i]) == SOLVED)
+		if (res[i] < 0)
+			continue;
+		if (check(&puzzles[i]) == SOLVED)
 			++solved;
+		tot_passes += (unsigned long)res[i];
 	}
+	const double succ_rate = (double)solved / NPUZZLES;
+	const unsigned avg_passes = tot_passes / NPUZZLES;
 
 	puts("\n       SUMMARY\n       =======");
-	printf("Success rate: %.0lf%%\n", 1e2 * (double)solved / NPUZZLES);
+	printf("Success rate: %.0lf%%\n", 1e2 * succ_rate);
 	printf("Average time: %.3lf µs\n", avg_micros);
+	printf("Average n.o. passes: %u\n", avg_passes);
 
 	return 0;
 }
diff --git a/solve.c b/solve.c
index ed9fb61..f762dd1 100644
--- a/solve.c
+++ b/solve.c
@@ -80,14 +80,14 @@ static enum apply_res apply_rules(struct sudoku *sud, cellgroup *group)
 	return matched ? MATCH : NO_MATCH;
 }
 
-bool solve(struct sudoku *sud)
+int solve(struct sudoku *sud)
 {
 	cellgroup group = { 0 };
-	unsigned r, c;
+	unsigned r, c, i, j, n;
 	bool match;
 	enum apply_res res;
 
-	for (unsigned i = 0;; ++i) {
+	for (n = 0;; ++n) {
 		match = false;
 
 		/* Apply rules to each row. */
@@ -100,7 +100,7 @@ bool solve(struct sudoku *sud)
 
 			res = apply_rules(sud, &group);
 			if (res == ERROR)
-				return false;
+				return -1;
 			else if (res == MATCH)
 				match = true;
 		}
@@ -115,7 +115,7 @@ bool solve(struct sudoku *sud)
 
 			res = apply_rules(sud, &group);
 			if (res == ERROR)
-				return false;
+				return -1;
 			else if (res == MATCH)
 				match = true;
 		}
@@ -124,6 +124,6 @@ bool solve(struct sudoku *sud)
 
 		/* Exit if no matches. */
 		if (!match)
-			return true;
+			return n;
 	}
 }
diff --git a/solve.h b/solve.h
index 1f65e7b..df47153 100644
--- a/solve.h
+++ b/solve.h
@@ -22,9 +22,10 @@
 #include "sud.h"
 
 /**
- * Attempt to solve the given sudoku. Returns when no rules match for
- * an entire pass over the puzzle.
+ * Attempt to solve the given sudoku. Finishes when no rules match for
+ * an entire pass over the puzzle, returning the number of passes, or
+ * -1 on error.
  */
-bool solve(struct sudoku *sud);
+int solve(struct sudoku *sud);
 
 #endif