diff --git a/number/exgcd.cc b/number/exgcd.cc
index 7a02a14..c1d276d 100644
--- a/number/exgcd.cc
+++ b/number/exgcd.cc
@@ -50,6 +50,7 @@ namespace Exgcd {
 
     // find the minimal non-negative integral solution of `ax = b (mod n)`
     optional<ll> congruential(ll a, ll b, ll n) {
+        if (a < 0 && a != LLONG_MIN && b != LLONG_MIN) a = -a, b = -b;
         auto sol = diophantine(a, n, b);
         if (sol == nullopt) {
             return nullopt;