diff --git a/pqcli/lingo.py b/pqcli/lingo.py
index 6731d6d..144a0ac 100644
--- a/pqcli/lingo.py
+++ b/pqcli/lingo.py
@@ -79,6 +79,43 @@ def to_roman(num: int) -> str:
     return ret
 
 
+def to_arabic(roman: str) -> int:
+    num = 0
+    rest = roman.upper()
+
+    def _arab(ds: str, dn: int) -> bool:
+        nonlocal num, rest
+        if not rest.startswith(ds):
+            return False
+        rest = rest[len(ds) :]
+        num += dn
+        return True
+
+    while _arab("T", 10000):
+        pass
+    _arab("MT", 9000)
+    _arab("A", 5000)
+    _arab("MA", 4000)
+    while _arab("M", 1000):
+        pass
+    _arab("CM", 900)
+    _arab("D", 500)
+    _arab("CD", 400)
+    while _arab("C", 100):
+        pass
+    _arab("XC", 90)
+    _arab("L", 50)
+    _arab("XL", 40)
+    while _arab("X", 10):
+        pass
+    _arab("IX", 9)
+    _arab("V", 5)
+    _arab("IV", 4)
+    while _arab("I", 1):
+        pass
+    return num
+
+
 def act_name(act: int) -> str:
     if act == 0:
         return "Prologue"
diff --git a/tests/test_lingo.py b/tests/test_lingo.py
index 584d6e5..5ca0562 100644
--- a/tests/test_lingo.py
+++ b/tests/test_lingo.py
@@ -1,4 +1,4 @@
-from pqcli.lingo import indefinite, to_roman
+from pqcli.lingo import indefinite, to_arabic, to_roman
 
 
 def test_indefinite_article() -> None:
@@ -19,3 +19,10 @@ def test_to_roman() -> None:
     assert to_roman(9000) == "MT"
     assert to_roman(14999) == "TMACMXCIX"
     assert to_roman(23456) == "TTMMMCDLVI"
+
+
+def test_to_arabic_inverts_to_roman() -> None:
+    for num in [*range(1, 5000), 9000, 9999, 14999, 23456]:
+        assert to_arabic(to_roman(num)) == num
+    assert to_arabic("N") == 0
+    assert to_arabic("iv") == 4