#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("inline")
#include<bits/stdc++.h>
using namespace std;
void*wmem;
char memarr[96000000];
template<class T> inline void walloc1d(T **arr, int x, void **mem = &wmem){
  static int skip[16] = {0, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
  (*mem) = (void*)( ((char*)(*mem)) + skip[((unsigned long long)(*mem)) & 15] );
  (*arr)=(T*)(*mem);
  (*mem)=((*arr)+x);
}
template<class T> inline void walloc1d(T **arr, int x1, int x2, void **mem = &wmem){
  walloc1d(arr, x2-x1, mem);
  (*arr) -= x1;
}
inline int my_getchar_unlocked(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread_unlocked(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite_unlocked(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar_unlocked(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite_unlocked(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar_unlocked(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
template<class S, class T> inline S chmax(S &a, T b){
  if(a<b){
    a=b;
  }
  return a;
}
struct graph{
  int N;
  int*es;
  int**edge;
  void setDirectEdge(int N__, int M, int A[], int B[], void **mem = &wmem){
    int i;
    N = N__;
    walloc1d(&es, N, mem);
    walloc1d(&edge, N, mem);
    walloc1d(&edge[0], M, mem);
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      es[A[i]]++;
    }
    for(i=(0);i<(N);i++){
      walloc1d(&edge[i], es[i], mem);
    }
    for(i=(0);i<(N);i++){
      es[i] = 0;
    }
    for(i=(0);i<(M);i++){
      edge[A[i]][es[A[i]]++] = B[i];
    }
  }
  int TopologicalSort(int res[], void *mem=wmem){
    int i;
    int j;
    int k;
    int rs;
    int*deg;
    int*q;
    int qs = 0;
    int qe = 0;
    walloc1d(&deg, N, &mem);
    walloc1d(&q, N, &mem);
    rs = 0;
    for(i=(0);i<(N);i++){
      deg[i] = 0;
    }
    for(i=(0);i<(N);i++){
      for(j=(0);j<(es[i]);j++){
        deg[edge[i][j]]++;
      }
    }
    for(i=(0);i<(N);i++){
      if(deg[i]==0){
        q[qe++] = i;
      }
    }
    while(qs < qe){
      i = q[qs++];
      res[rs++] = i;
      for(j=(0);j<(es[i]);j++){
        k = edge[i][j];
        deg[k]--;
        if(deg[k]==0){
          q[qe++] = k;
        }
      }
    }
    if(rs==N){
      return 1;
    }
    return 0;
  }
  int longestPath_length(void *mem = wmem){
    int*dp;
    int*arr;
    int res = 0;
    int i;
    int j;
    int ii;
    int jj;
    walloc1d(&dp, N, &mem);
    walloc1d(&arr, N, &mem);
    if(!TopologicalSort(arr, mem)){
      return -1;
    }
    for(i=(0);i<(N);i++){
      dp[i] = 0;
    }
    for(ii=(0);ii<(N);ii++){
      i = arr[ii];
      chmax(res, dp[i]);
      for(jj=(0);jj<(es[i]);jj++){
        j = edge[i][jj];
        chmax(dp[j], dp[i] + 1);
      }
    }
    return res;
  }
}
;
int N;
int K;
int A[100000];
int B[100000];
graph g;
int main(){
  wmem = memarr;
  int res;
  rd(N);
  rd(K);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=(0);Lj4PdHRW<(K);Lj4PdHRW++){
      rd(A[Lj4PdHRW]);A[Lj4PdHRW] += (-1);
      rd(B[Lj4PdHRW]);B[Lj4PdHRW] += (-1);
    }
  }
  g.setDirectEdge(N,K,A,B);
  if(K==0){
    wt_L(1);
    wt_L('\n');
    return 0;
  }
  res = g.longestPath_length();
  if(res==-1){
    wt_L(res);
    wt_L('\n');
    return 0;
  }
  wt_L(res+1);
  wt_L('\n');
  return 0;
}
// cLay version 20210819-1 [beta]

// --- original code ---
// int N, K, A[1d5], B[1d5];
// graph g;
// {
//   int res;
//   rd(N,K,(A--,B--)(K));
//   g.setDirectEdge(N,K,A,B);
//   if(K==0) wt(1), return 0;
//   res = g.longestPath_length();
//   if(res==-1) wt(res), return 0;
//   wt(res+1);
// }