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

void main()
{
    auto N = readln.strip.to!int;
    long[] a, b;
    foreach (_; 0 .. N)
    {
        auto input = readln.split.to!(long[]);
        a ~= input[0];
        b ~= input[1];
    }

    auto widths = zip(a, b).map!"a[0]+4*a[1]";
    auto diffs  = widths.map!(diff => widths.minPos!"a>b".front-diff);
    (diffs.filter!"a%2==1".empty ? diffs.sum / 2 : -1).writeln;
}