import std.stdio, std.conv, std.math, std.string, std.range, std.array,
       std.algorithm;

void main() {
  immutable N = readln().strip().to!int();

  int[] a;
  a.length = N;
  
  foreach(immutable int i; 0 .. N) {
    auto buf = readln().strip().split().map!(to!int)().array();
    if(buf[0]%2 == 1) {
      writeln(-1);
      return;
    }
    a[i] = buf[0]/2 + buf[1]*2;
  }

  (a.reduce!max * N - a.reduce!"a+b").writeln();

}