diff --git a/src/main.odin b/src/main.odin
index a851bb5..859cc06 100644
--- a/src/main.odin
+++ b/src/main.odin
@@ -1,6 +1,7 @@
 package main
 
 import "core:fmt"
+import "core:math"
 import "core:os"
 import "core:strconv"
 import "core:strings"
@@ -27,6 +28,28 @@ read_data :: proc(file: string) -> (res: [NUMBER_OF_ITEMS]Item, ok := true) {
 	return
 }
 
+Chromosome :: [NUMBER_OF_ITEMS]bool
+
+distance :: proc(x, y: int) -> int {
+	return math.abs(x - y)
+}
+
+penalize :: proc(d: int) -> int {
+	return math.min(-3 * d, 0)
+}
+
+// side-effect: reads global `items`
+fitness :: proc(chrom: Chromosome) -> int {
+	tot_profit, tot_weight := 0, 0
+	for bit, idx in chrom {
+		if !bit {continue}
+		tot_profit += items[idx].profit
+		tot_weight += items[idx].weight
+	}
+	return tot_profit + penalize(distance(tot_weight, CAPACITY))
+}
+
+
 main :: proc() {
 	items, ok := read_data(DATA_FILE)
 	if !ok {