#include<cstdio>
#include <algorithm>
#include <cstdlib>
#include <map>
using namespace std;
int N, A[101010], B[101010];
void fail()
{
puts("-1");
exit(0);
}
int bas[101010];
map<pair<int, int>, int> P;
bool is_good(int y, int x)
{
return P.count(make_pair(y - 1, x - 1)) > 0;
}
int main()
{
scanf("%d", &N);
for (int i = 0; i < N; ++i) scanf("%d%d", A + i, B + i);
if (A[0] != 0) fail();
if (B[1] != 0) fail();
if (A[1] != B[0]) fail();
int D = A[1];
for (int i = 2; i < N; ++i) {
if (A[i] == 0 || B[i] == 0 || A[i] + B[i] < D) fail();
}
for (int i = 0; i < N; ++i) {
if (A[i] + B[i] == D) {
bas[A[i]] += 1;
}
}
int ret = 0;
for (int i = 0; i < D; ++i) {
if (bas[i] == 0) fail();
ret += max(bas[i], bas[i + 1]);
}
for (int i = 0; i < N; ++i) P[make_pair(A[i], B[i])] += 1;
for (int i = 2; i < N; ++i) if (A[i] + B[i] != D) {
int x = A[i], y = B[i];
if (P.count(make_pair(x - 1, y - 1))) {
++ret;
} else if (P.count(make_pair(x, y - 1)) && P.count(make_pair(x - 1, y))) {
ret += 2;
} else fail();
}
for (map<pair<int, int>, int>::iterator p = P.begin(); p != P.end(); ++p) {
int y = p->first.first, x = p->first.second;
if (y + x == D) continue;
if (P.count(make_pair(y, x)) && P.count(make_pair(y - 1, x + 1)) && !is_good(y, x) && !is_good(y - 1, x + 1)) {
ret -= min(P[make_pair(y, x)], P[make_pair(y - 1, x + 1)]);
}
}
printf("%d\n", ret);
return 0;
}
./Main.cpp: In function ‘int main()’:
./Main.cpp:25:17: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
scanf("%d", &N);
^
./Main.cpp:26:57: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
for (int i = 0; i < N; ++i) scanf("%d%d", A + i, B + i);
^