diff --git a/Cargo.lock b/Cargo.lock
index fb46a6c..2706aa5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4,4 +4,4 @@ version = 3
 
 [[package]]
 name = "electosim"
-version = "0.2.2"
+version = "0.2.3"
diff --git a/Cargo.toml b/Cargo.toml
index bd471d1..33e038a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "electosim"
-version = "0.2.2"
+version = "0.2.3"
 edition = "2021"
 authors = ["Eduardo González Vaquero <edugonzalezvaq@gmail.com>"]
 description = "Library to compute electoral methods (as D'Hondt) and simulate elections"
diff --git a/src/interface.rs b/src/interface.rs
index 44cffee..cb7e83d 100644
--- a/src/interface.rs
+++ b/src/interface.rs
@@ -5,6 +5,16 @@ pub trait WithVotes {
 
     /// Sets the number of votes.
     fn set_votes(&mut self, votes: u32);
+
+    /// Increases the number of votes by a given amount.
+    fn increase_votes(&mut self, n: i32) {
+        let current_votes = self.get_votes();
+        if n < 0 {
+            self.set_votes(current_votes.saturating_sub(n.abs() as u32));
+        } else {
+            self.set_votes(current_votes + n as u32);
+        }
+    }
 }
 
 /// A trait for objects that have seats.
@@ -16,9 +26,13 @@ pub trait WithSeats {
     fn set_seats(&mut self, seats: u16);
 
     /// Increases the number of seats by a given amount.
-    fn increase_seats(&mut self, n: u16) {
-        let actual_seats = self.get_seats();
-        self.set_seats(actual_seats + n);
+    fn increase_seats(&mut self, n: i16) {
+        let current_seats = self.get_seats();
+        if n < 0 {
+            self.set_seats(current_seats.saturating_sub(n.abs() as u16));
+        } else {
+            self.set_seats(current_seats + n as u16);
+        }
     }
 }
 
@@ -54,6 +68,7 @@ mod tests {
 
     struct Candidate {
         votes: u32,
+        seats: u16,
     }
 
     impl WithVotes for Candidate {
@@ -66,13 +81,66 @@ mod tests {
         }
     }
 
+    impl WithSeats for Candidate {
+        fn get_seats(&self) -> u16 {
+            self.seats
+        }
+
+        fn set_seats(&mut self, seats: u16) {
+            self.seats = seats;
+        }
+    }
+
     #[test]
     fn test_with_votes() {
-        let mut candidate = Candidate { votes: 1000 };
+        let mut candidate = Candidate {
+            votes: 1000,
+            seats: 0,
+        };
+
+        assert_eq!(candidate.get_votes(), 1000);
+        candidate.set_votes(2000);
+        assert_eq!(candidate.get_votes(), 2000);
+
+        candidate.increase_votes(100);
+        assert_eq!(candidate.get_votes(), 2100);
+
+        candidate.increase_votes(-200);
+        assert_eq!(candidate.get_votes(), 1900);
+    }
+
+    #[test]
+    fn test_with_seats() {
+        let mut candidate = Candidate {
+            votes: 1000,
+            seats: 50,
+        };
+
+        assert_eq!(candidate.get_seats(), 50);
+        candidate.set_seats(100);
+        assert_eq!(candidate.get_seats(), 100);
+
+        candidate.increase_seats(10);
+        assert_eq!(candidate.get_seats(), 110);
+
+        candidate.increase_seats(-20);
+        assert_eq!(candidate.get_seats(), 90);
+    }
+
+    #[test]
+    fn test_with_box() {
+        let mut candidate = Candidate {
+            votes: 1000,
+            seats: 50,
+        };
+
         let mut boxed_candidate = Box::new(&mut candidate);
 
         assert_eq!(boxed_candidate.get_votes(), 1000);
         boxed_candidate.set_votes(2000);
-        assert_eq!(candidate.get_votes(), 2000);
+        assert_eq!(boxed_candidate.get_votes(), 2000);
+
+        boxed_candidate.set_seats(100);
+        assert_eq!(boxed_candidate.get_seats(), 100);
     }
 }