import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N = int(readline())
m = map(int,read().split())
AB = zip(m,m)

diff = [b-a for a,b in AB]

if len(set(diff)) > 1:
    print(-1)
else:
    x = diff[0]
    if x <= 0:
        print(-1)
    else:
        print(x)