#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, vector<pair<int, int>>> s;
for (int i = 0; i < n; i ++) {
s[p[i].first + p[i].second].emplace_back(p[i].first, p[i].second);
}
int matching = 0;
for (auto mp : s) {
vector<pair<int, int>> v = mp.second;
sort(v.begin(), v.end());
int cur = v[0].first;
vector<pair<bool, pair<int, int>>> t; //(bottom left exists, number of the same position)
for (int i = 0; i < (int) v.size(); i ++) {
int cnt = 0;
bool ex = ps.count({v[i].first - 1, v[i].second - 1});
while (i < (int) v.size() && cur == v[i].first) {
i ++;
cnt ++;
}
i --;
t.push_back({ex, {cnt, cnt}});
cur ++;
}
for (int i = 0; i < (int) t.size(); i ++) {
if (t[i].first) {
int d = min(t[i].second.first, t[i].second.second);
matching += d;
t[i].second.first -= d;
t[i].second.second -= d;
}
if (t[i].second.second > 0 && i + 1 < (int) t.size()) {
int d = min(t[i].second.second, t[i + 1].second.first);
matching += d;
t[i + 1].second.first -= d;
}
}
}
printf("%d\n", 2 * n - 2 - matching);
return 0;
}