import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();
		
		int[] X = new int[N], Y = new int[N];
		
		for(int i = 0; i < N; i++) {
			X[i] = sc.nextInt();
			Y[i] = sc.nextInt();
		}
		
		int[] Z = new int[N];
		
		for(int i = 0; i < N; i++) {
			Z[i] = Y[i] - X[i];
		}
		
		int cnt = 0;
		
		for(int i = 0; i < N; i++) {
			if(Z[0] == Z[i] && Z[i] >= 0) {
				cnt++;
			}
		}
		
		if(cnt == N) {
			System.out.println(cnt);
		} else {
			System.out.println(-1);
		}
	}
}