#include <stdio.h>
#include <string.h>

void main(void) {
	int n = 0;
	scanf("%d",&n);
	int numA,numB;
	int resultList[n];
	for(int i = 0 ; i < n ; i++ ){
		scanf("%d %d",&numA,&numB);
		resultList[i] = numB - numA;
	}
	int x = resultList[0];
	if(x >= 0 && x <= n){	
		for(int i = 1 ; i < n ; i++ ){	
			if(resultList[i] < 0 || resultList[i] > n || resultList[i] != x){
				x = -1;
				break;
			}
		}
	}else{
		x = -1;
	}
	printf("%d",x);
}