diff --git a/MFMM.md b/MFMM.md
index 9c7f38d..f32365c 100644
--- a/MFMM.md
+++ b/MFMM.md
@@ -174,7 +174,7 @@ acc0 - H(acc0 * 2^32) >= 0显然。
 	ADCQ acc0, acc4
 	ADCQ $0, acc5
 	SUBQ AX, acc1
-	SUBQ DX, acc2
+	SBBQ DX, acc2
 	SBBQ AX, acc3
 	SBBQ DX, acc4
 	SBBQ $0, acc5