diff --git a/src/bignum/bignum.cc b/src/bignum/bignum.cc
index 473af47..c502a59 100644
--- a/src/bignum/bignum.cc
+++ b/src/bignum/bignum.cc
@@ -27,6 +27,12 @@ BigNum::BigNum(std::int64_t number) {
     assert(is_canonicalized());
 }
 
+void BigNum::flip_sign() {
+    assert(is_canonicalized());
+
+    sign_ *= -1;
+}
+
 bool BigNum::equal(BigNum const& rhs) const {
     assert(is_canonicalized());
     assert(rhs.is_canonicalized());
diff --git a/src/bignum/bignum.hh b/src/bignum/bignum.hh
index 110c3c4..ad95bd2 100644
--- a/src/bignum/bignum.hh
+++ b/src/bignum/bignum.hh
@@ -10,6 +10,16 @@ class BigNum {
 public:
     explicit BigNum(std::int64_t number);
 
+    friend BigNum operator+(BigNum const& rhs) {
+        return rhs;
+    }
+
+    friend BigNum operator-(BigNum const& rhs) {
+        auto ret = rhs;
+        ret.flip_sign();
+        return ret;
+    }
+
     friend bool operator==(BigNum const& lhs, BigNum const& rhs) {
         return lhs.equal(rhs);
     }
@@ -35,6 +45,8 @@ public:
     }
 
 private:
+    void flip_sign();
+
     bool equal(BigNum const& rhs) const;
     bool less_than(BigNum const& rhs) const;
 
diff --git a/tests/unit/bignum.cc b/tests/unit/bignum.cc
index e7dfdc6..cff20c2 100644
--- a/tests/unit/bignum.cc
+++ b/tests/unit/bignum.cc
@@ -25,3 +25,14 @@ TEST(BigNum, comparisons) {
     ASSERT_GE(one, zero);
     ASSERT_GE(one, one);
 }
+
+TEST(BigNum, unary) {
+    auto const zero = BigNum(0);
+    auto const one = BigNum(1);
+
+    ASSERT_EQ(zero, -zero);
+    ASSERT_NE(one, -one);
+
+    auto const minus_one = BigNum(-1);
+    ASSERT_EQ(minus_one, -one);
+}