diff --git a/app-common/src/main/java/im/angry/openeuicc/ui/wizard/DownloadWizardDetailsFragment.kt b/app-common/src/main/java/im/angry/openeuicc/ui/wizard/DownloadWizardDetailsFragment.kt
index 88567c2..69ed77f 100644
--- a/app-common/src/main/java/im/angry/openeuicc/ui/wizard/DownloadWizardDetailsFragment.kt
+++ b/app-common/src/main/java/im/angry/openeuicc/ui/wizard/DownloadWizardDetailsFragment.kt
@@ -87,7 +87,13 @@ private fun isValidIMEI(input: CharSequence): Boolean {
 
     fun sumOfDigits(input: Int): Int {
         if (input % 2 == 0) return input
-        return (input * 2).toString().map(Char::digitToInt).sum()
+        var n = input
+        var sum = 0
+        while (n > 0) {
+            sum += n % 10
+            n /= 10
+        }
+        return sum
     }
 
     val sum = input.dropLast(1).map(Char::digitToInt).sumOf(::sumOfDigits)