let inputCount = Int(readLine()!)!

var diff:Int = -1
for i in 0..<inputCount {
    let inpt = readLine()!.split(separator:" ").map{Int($0)!}
    if(i==0) {
        diff = inpt[1]-inpt[0]
    } else {
        if(diff + inpt[0] != inpt[1]) {
            diff = -1
            break
        }
    }
}

if(diff <= 0) {
    diff = -1
}
print(diff)