#include <stdio.h>

int main()
{
	int N, result = -2;
	scanf("%d", &N);

	for (int i = 0; i < N; i++)
	{
		int X, Y;
		scanf("%d%d", &X, &Y);

		if (result == -1)
		{
			continue;
		}

		if (result == -2)
		{
			if (X < Y)
			{
				result = Y - X;
			}
			else
			{
				result = -1;
			}
			continue;
		}

		if (result != Y - X)
		{
			result = -1;
		}

	}

	printf("%d", result);

	return 0;
}