#include "game.h"
#include<bits/stdc++.h>
using namespace std;
const int N = 30'000 + 10, K = 1000 + 10;
bitset<N> f[K];
vector<int> adj[N];
bool cycle;
int n, k;
void expand (int root, int u) {
f[root][u] = 1;
for(int &v : adj[u]) {
if (v == root) cycle = 1;
if (!f[root][v]) expand(root, v);
}
}
void add (int u, int v) {
adj[u].push_back(v);
for(int start = 0; start < k; start++) {
if (f[start][u] && v == start) cycle = 1;
if (f[start][u] && !f[start][v]) expand(start, v);
}
}
void init(int N, int K) {
n = N;
k = K;
cycle = 0;
for(int i = 0; i < k; i++) f[i][i] = 1;
for(int i = 0; i + 1 < k; i++) add(i, i + 1);
}
int add_teleporter(int u, int v) {
add(u, v);
return cycle;
}
#ifdef ngu
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 ("task.inp", "r", stdin);
freopen ("task.out", "w", stdout);
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 // ngu
