#include <iostream>
#include <fstream>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <bitset>
#include <algorithm>
#include <cmath>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <chrono>
#include <random>
#include <complex>
#include <numeric>
#include <assert.h>
#include <functional>
#include <climits>
#include <cstring>
#include <iterator>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
using ll = long long;
using ld = long double;
using ushort = unsigned short;
#ifndef LOCAL
#define endl "\n"
#endif
#define f first
#define s second
#define vec vector
#define pii pair<int, int>
#define all(x) x.begin(), x.end()
#define pb push_back
#define mp make_pair
template<typename T1, typename T2>
istream& operator>> (istream &in, pair<T1, T2> &p) { in >> p.f >> p.s; return in; }
template<typename T1, typename T2>
ostream& operator<< (ostream &out, pair<T1, T2> p) { out << "(" << p.f << " " << p.s << ")"; return out; }
#define printv(v) cerr << #v << " "; for (auto &x : v) { cerr << x << " "; } cerr << endl;
#define printvv(v) cerr << #v << " "; for (auto &x : v) { for (auto &y : x) { cerr << y << " "; } cerr << endl; }
#define debug(x) cerr << #x << " " << x << endl;
template<typename T>
istream& operator>> (istream &in, vector<T> &v) {
for (auto &x : v) {
in >> x;
}
return in;
}
template<typename T>
ostream& operator<< (ostream &out, vector<T> v) {
for (auto &x : v) {
out << x << " ";
}
return out;
}
template<typename T>
void operator-=(vector<T> &v, int delta) {
for (auto &x : v) {
x -= delta;
}
}
template<typename T>
void operator+=(vector<T> &v, int delta) {
for (auto &x : v) {
x += delta;
}
}
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
vector<vector<int>> graph, rgraph;
bool answer;
vector<int> l, r, nl, nr;
int K;
void init(int n, int k) {
l.assign(n, 0);
r.assign(n, k + 1);
nl.assign(n, 0);
nr.assign(n, k + 1);
K = k;
graph.assign(n, {});
rgraph.assign(n, {});
answer = false;
for (int i = 0; i < k; ++i) {
l[i] = i + 1;
r[i] = i + 1;
nl[i] = i + 1;
nr[i] = i + 1;
}
}
void update(int v) {
if (v < K) {
return;
}
if (answer) {
return;
}
while (true) {
int m = (l[v] + r[v]) / 2;
if (nl[v] > m || nr[v] <= m) {
l[v] = max(l[v], m);
r[v] = min(r[v], m);
if (l[v] >= r[v]) {
answer = true;
}
for (auto &u: graph[v]) {
nl[u] = max(nl[u], l[v]);
update(u);
}
for (auto &u: rgraph[v]) {
nr[u] = min(nr[u], r[v]);
update(u);
}
} else {
break;
}
}
}
int add_teleporter(int a, int b) {
if (answer) {
return true;
}
if (a < K && b < K) {
if (a >= b) {
answer = true;
return true;
}
}
graph[a].pb(b);
rgraph[b].pb(a);
if (b >= K) {
if (nl[b] < l[a]) {
nl[b] = l[a];
update(b);
}
}
if (a >= K) {
if (nr[a] > r[b]) {
nr[a] = r[b];
update(a);
}
}
return answer;
}
#ifdef LOCAL
namespace {
int read_int() {
int x;
if (scanf("%d", &x) != 1) {
fprintf(stderr, "Error while reading input\n");
exit(1);
}
return x;
}
} // namespace
int main() {
freopen("in.txt", "r", stdin);
int N = read_int();
int M = read_int();
int K = read_int();
std::vector<int> u(M), v(M);
for (int i = 0; i < M; ++i) {
u[i] = read_int();
v[i] = read_int();
}
init(N, K);
int i;
for (i = 0; i < M; ++i) {
int answer = add_teleporter(u[i], v[i]);
if (answer != 0 && answer != 1) {
i = -1;
break;
} else if (answer == 1) {
break;
}
}
printf("%d\n", i);
return 0;
}
#endif
