#include <cstdio>
#include <vector>
#include <algorithm>
#include <functional>
#include <map>
#include <set>
#include <string>
#include <iostream>
#include <cassert>
#include <cmath>
using namespace std;
void ng() {
printf("-1\n");
exit(0);
}
int main() {
int n;
scanf("%d", &n);
set<pair<int, int>> ps;
vector<pair<int, int>> p;
for (int i = 0; i < n; i ++) {
int a, b;
scanf("%d%d", &a, &b);
p.emplace_back(a, b);
ps.insert({a, b});
}
if (p[0].first != 0 || p[1].second != 0) ng();
if (p[0].second != p[1].first) ng();
if (p[0].second == 0) ng();
for (int i = 2; i < n; i ++) {
int a, b;
tie(a, b) = p[i];
if (a == 0 || b == 0) ng();
if (!ps.count({a - 1, b + 1}) && !ps.count({a - 1, b}) && !ps.count({a - 1, b - 1})) ng();
if (!ps.count({a + 1, b - 1}) && !ps.count({a, b - 1}) && !ps.count({a - 1, b - 1})) ng();
}
map<int, map<int, int>> s;
for (int i = 0; i < n; i ++) {
s[p[i].first + p[i].second][p[i].first] ++;
}
int matching = 0;
for (auto mpmp : s) {
auto sum = mpmp.first;
auto mp = mpmp.second;
vector<pair<int, int>> v;
for (auto it : mp) {
v.emplace_back(it.first, it.second);
}
sort(v.begin(), v.end());
vector<vector<pair<bool, pair<int, int>>>> t; //(bottom left exists, number of the same position)
vector<pair<bool, pair<int, int>>> tmp; //(bottom left exists, number of the same position)
int preva = -1;
for (auto u : v) {
int a = u.first;
if (preva != -1 && preva + 1 != a) {
t.push_back(tmp);
tmp.clear();
}
int b = sum - a;
bool ex = ps.count({a - 1, b - 1});
tmp.push_back({ex, {u.second, u.second}});
preva = a;
}
if (tmp.size() > 0) t.push_back(tmp);
for (auto it : t) {
for (int i = 0; i < (int) it.size(); i ++) {
if (it[i].first) {
int d = min(it[i].second.first, it[i].second.second);
matching += d;
it[i].second.first -= d;
it[i].second.second -= d;
}
if (it[i].second.second > 0 && i + 1 < (int) it.size()) {
int d = min(it[i].second.second, it[i + 1].second.first);
matching += d;
it[i + 1].second.first -= d;
}
}
}
}
printf("%d\n", 2 * n - 2 - matching);
return 0;
}