import std.stdio;
import std.algorithm;
import std.conv;
import std.array;
import std.string;

void main()
{
    auto n = readln().chomp.to!uint;
    uint max = 0;
    uint[] lines;
    foreach (i; 0..n) {
        auto input = readln().split().map!(to!uint);
        auto l = input[0] + input[1] * 4;
        lines ~= l;
        if (max < l)
            max = l;
    }
    uint cnt = 0;
    foreach (l; lines) {
        auto z = max - l;
        if (z % 2 != 0) {
            writeln(-1);
            return;
        }
        cnt += z / 2;
    }
    writeln(cnt);
}