import sys; input = sys.stdin.buffer.readline
sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	N = int(input())
	L = []
	for i in range(N):
		a, b = getlist()
		val = a + 4 * b
		L.append(val)

	L.sort()
	ans = 0
	for i in range(N):
		if (L[-1] - L[i]) % 2 == 1:
			print(-1)
			return
		else:
			ans += int((L[-1] - L[i]) // 2)

	print(ans)


if __name__ == '__main__':
	main()