import java.util.*;

class No0185{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		long n  = sc.nextLong();
		long result = 0;
		ArrayList<Long> x = new ArrayList<Long>();

		for(int i = 0; i < n; i++){
			x.add(sc.nextLong());
			x.set(i, (sc.nextLong() - x.get(i)));
		}

		for(int i = 0; i < n; i++){
			if(x.get(i) < 0){
				result = -1;
				break;
			}
			if(x.get(i) != x.get(i + 1)){
				result = -1;
				break;
			}
			if(i == n - 2){
				break;
			}
		}
		
		if(result != -1) result = x.get(0);
		System.out.println(result);
	}
}