// yukicoder: No.178 美しいWhitespace (1)
// 2019.4.13 bal4u

#include <stdio.h>

#if 1
#define gc() getchar_unlocked()
#else
#define gc() getchar()
#endif
int in()
{
	int n = 0, c = gc();
	do n = 10 * n + (c & 0xf), c = gc(); while (c >= '0');
	return n;
}

int N;
int w[1005];

int main()
{
	int i, a, b, max;
	long long ans;

	N = in();
	max = 0, ans = 0;
	for (i = 0; i < N; i++) {
		a = in(), b = in();
		w[i] = a + (b << 2);
		if (w[i] > max) max = w[i];
	}
	for (i = 0; i < N; i++) {
		if ((max - w[i]) & 1) {	puts("-1"); return 0; }
		ans += (max - w[i]) >> 1;
	}
	printf("%lld\n", ans);
	return 0;
}