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

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

  long[] a;
  a.length = N;
  
  foreach(immutable int i; 0 .. N) {
    auto buf = readln().strip().split().map!(to!int)().array();
    a[i] = buf[0] + buf[1]*4;
  }

  if(a.all!"a%2 == 1" || a.all!"a%2 == 0") {
    ((a.reduce!max * N - a.reduce!"a+b")/2).writeln();
  } else {
    writeln(-1);
  }

}