#include <stdio.h>
#include <malloc.h>

char str[60000];
int str_len = 0;

/// <summary>
/// グローバル変数strの中身を入力された文字列で上書きする
/// </summary>
void ReadString() {
	char c = getchar();
	str_len = 0;

	while (c != '\n') {
		str[str_len] = c;
		c = getchar();
		str_len++;
	}
}

/// <summary>
/// 入力された数字を返す
/// </summary>
/// <returns></returns>
int ReadNum() {
	int negate = 0;
	char c = getchar();
	int num = 0;
	int numCnt = 0;
	while (c != '\n') {
		if (c == '-') {
			negate = 1;
		}
		else {
			num = num * 10 + c - '0';
		}

		c = getchar();
	}
	if (negate == 1) {
		num *= -1;
	}
	return num;
}

/// <summary>
/// ReadStringの文字列のうち、スペースで区切られた左側か右側を取得する
/// </summary>
/// <param name="leftIs">左が欲しければtrue,右ならfalse</param>
/// <returns>引数に対応した側の数字を返す</returns>
int GetLR(int leftIs) {
	int cnt = 0;
	int num = 0;
	int left = 0;
	int right = 0;
	while (1) {
		if (cnt >= str_len) {
			right = num;
			break;
		}
		if (str[cnt] == ' ') {
			left = num;
			num = 0;
			cnt++;
			continue;
		}
		num = num * 10 + (str[cnt] - '0');
		cnt++;
	}
	if (!leftIs) {
		return left;
	}
	else {
		return right;
	}
}

int getWafu() {
	int n = ReadNum() - 1;
	ReadString();
	if (GetLR(1) - GetLR(0) <= 0) return -1;
	int ans = GetLR(1) - GetLR(0);
	int wafu = ans;
	for (int i = 0;i < n;i++) {
		ReadString();
		if (GetLR(1) - GetLR(0) != ans) wafu = -1;
	}
	return wafu;
}


int main()
{
	printf("%d\n",getWafu());
}