diff --git a/day-5/two.go b/day-5/two.go
index 1fc2ed6..aae550e 100644
--- a/day-5/two.go
+++ b/day-5/two.go
@@ -6,6 +6,7 @@ import (
 )
 
 var wg sync.WaitGroup
+var mu sync.Mutex
 
 const UNLIMITED = -1
 
@@ -68,7 +69,9 @@ func two(lines []string) int {
 		wg.Add(1)
 		go func(i int) {
 			defer wg.Done()
+			mu.Lock()
 			seedRanges = append(seedRanges, nextTransform(0, []bound{newBound(seeds[i], seeds[i]+seeds[i+1]-1)})...)
+			mu.Unlock()
 		}(i)
 	}