// yukicoder: No.185 和風
// 2019.4.13 bal4u

#include <stdio.h>

//// 高速入力
#if 1
#define gc() getchar_unlocked()
#define pc(c) putchar_unlocked(c)
#else
#define gc() getchar()
#define pc(c) putchar(c)
#endif
int in()    // 非負整数の入力
{
	int n = 0, c = gc();
	do n = 10 * n + (c & 0xf), c = gc(); while (c >= '0');
	return n;
}

int main()
{
	int N, x, ans;

	N = in();
	x = in(), ans = in() - x;
	if (ans <= 0) ans = -1;
	if (ans > 0) while (--N) {
		x = in();
		if (in() - x != ans) { ans = -1; break; }
	}
	printf("%d\n", ans);
	return 0;
}