import java.util.Scanner;

class Test {
	public static void main(String[] args) throws Exception {
		Scanner in = new Scanner(System.in);
		int n = in.nextInt();

                int a = 0, b = 0;
		int[] w = new int[n];
		int max = 0;
		boolean bool = true;

		for(int i=0; i<n; i++) {
			a = in.nextInt();
			b = in.nextInt();
			w[i] = a + b * 4;
			max = Math.max(max, w[i]);
			if(w[i] % 2 != w[i-1] % 2) {
				bool = false;
			}
		}

		if(bool) {
			int ans = 0;
			for(int i=0; i<n; i++) {
				ans += (max - w[i]) / 2;
			}
			System.out.println(ans);
		} else {
			System.out.println(-1);
		}
	}
}