import std.algorithm, std.array, std.container, std.range, std.bitmanip;
import std.numeric, std.math, std.bigint, std.random;
import std.string, std.conv, std.stdio, std.typecons;

void main()
{
  auto n = readln.chomp.to!int;
  auto ai = iota(n)
    .map!(_ => readln.split.map!(to!int))
    .map!(rd => rd[1] - rd[0]).array;
  
  if (ai.any!(a => a <= 0) || ai.length >= 2 && ai[1..$].any!(a => a != ai.front)) {
    writeln(-1);
  } else {
    writeln(ai.front);
  }
}