from decimal import *
import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]

def main():
    n=int(input())
    x,y=MI()
    d=y-x
    if d<=0:
        print(-1)
        exit()
    for _ in range(n-1):
        x,y=MI()
        if y-x!=d:
            print(-1)
            exit()
    print(d)

main()