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diff --git a/2020/day3/README.md b/2020/day3/README.md
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+# day 3
+
+## first part
+
+32 places per row, right 3 and down 1 is
+
+```
+idx = (idx + 3) % 32
+```
+
+per line but this will _probably_ change for the part 2.