import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


int N, M;
int[] A, B;

int I;
int[][] G;
int[] vis;

bool dfs(int u) {
  vis[u] = 1;
  foreach (i; G[u]) if (i < I) {
    const v = B[i];
    if (vis[v] == 0) {
      if (!dfs(v)) {
        return false;
      }
    } else if (vis[v] == 1) {
      return false;
    }
  }
  vis[u] = 2;
  return true;
}

bool check(int I_) {
  I = I_;
  vis = new int[N];
  foreach (u; 0 .. N) if (vis[u] == 0) {
    if (!dfs(u)) {
      return false;
    }
  }
  return true;
}

void main() {
  try {
    for (; ; ) {
      N = readInt;
      M = readInt;
      A = new int[M];
      B = new int[M];
      foreach (i; 0 .. M) {
        A[i] = readInt - 1;
        B[i] = readInt - 1;
      }
      
      G = new int[][N];
      foreach (i; 0 .. M) {
        G[A[i]] ~= i;
      }
      
      int lo = 0, hi = M + 1;
      for (; lo + 1 < hi; ) {
        const mid = (lo + hi) / 2;
        (check(mid) ? lo : hi) = mid;
      }
      writeln((hi == M + 1) ? -1 : hi);
    }
  } catch (EOFException e) {
  }
}