#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx,popcnt,sse4,abm")
#include <bits/stdc++.h>
using namespace std;
#ifndef WAIMAI
#include "game.h"
#endif
#ifdef WAIMAI
#define debug(HEHE...) cout << "[" << #HEHE << "] : ", dout(HEHE)
void dout() {cout << '\n';}
template<typename T, typename...U>
void dout(T t, U...u) {cout << t << (sizeof...(u) ? ", " : ""), dout(u...);}
#else
#define debug(...) 7122
#endif
#define ll long long
#define Waimai ios::sync_with_stdio(false), cin.tie(0)
#define FOR(x,a,b) for (int x = a, I = b; x <= I; x++)
#define pb emplace_back
#define F first
#define S second
const int SIZE = 3e5 + 5;
int n, k;
int l[SIZE], r[SIZE];
vector<int> adj[SIZE], rev[SIZE];
void init(int N, int K) {
n = N, k = K;
FOR (i, 0, k - 1) l[i] = r[i] = i;
FOR (i, k, n - 1) l[i] = -1, r[i] = k;
}
bool upd(int a, int b) {
if (l[a] >= r[b]) return 1;
int mid = (l[a] + r[a]) >> 1;
if (r[b] <= mid) {
r[a] = mid;
for (int p : adj[a]) if (upd(a, p)) return 1;
for (int p : rev[a]) if (upd(p, a)) return 1;
}
mid = (l[b] + r[b]) >> 1;
if (l[a] > mid) {
l[b] = mid + 1;
for (int p : adj[b]) if (upd(b, p)) return 1;
for (int p : rev[b]) if (upd(p, b)) return 1;
}
return 0;
}
int add_teleporter(int a, int b) {
if (max(a, b) < k && a >= b) return 1;
adj[a].pb(b);
rev[b].pb(a);
return upd(a, b);
}
#ifdef WAIMAI
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() {
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
