import sequtils,strutils
var
    N = stdin.readline.parseInt
    x,y : int
    z = 0
for i in 1..N:
    (x, y) = stdin.readline.split.map(parseInt)
    if y - x < 1:
        echo -1
        quit()
    if i == 1:
        z = y - x
    elif z == y - x:
        continue
    else:
        echo -1
        quit()
echo z