#include<iostream>

#include<cstdio>

#include<cstring>

#include<cmath>

#include<algorithm>

#include<string.h>

#define maxint 999999999

#define MAXN 10005

#define ll long long

using namespace std;

int n,m,k,flag,num;

ll sum;

int Mark[MAXN],st[MAXN],vis[MAXN];

struct node{

     int x,y;

     int flag;

}cmp[];

void topsort(){

    int l=;

    while(){

        int j;

        int s=;

        for(j=;j<=n;j++){

            if(Mark[j]==){

                sum+=num;

                --Mark[j];

                ++s;

                vis[s]=j;

                ++l;

            }

        }

        num++;

        if(l==n)

           break;

        if(!s){

            flag=;

            break;

        }

        else{

            for(int i=;i<=s;i++){

            for(int k=;k<=m;k++){

                if(cmp[k].y==vis[i]&&Mark[cmp[k].x]!=&&cmp[k].flag==){

                    --Mark[cmp[k].x];

                    cmp[k].flag=;

                }

            }

            }

        }

    }

}

int main()

{

    while(scanf("%d %d",&n,&m)!=EOF){

        sum=;

        num=;

        memset(vis,,sizeof(vis));

        memset(Mark,,sizeof(Mark));

        for(int i=;i<=m;i++){

            scanf("%d %d",&cmp[i].x,&cmp[i].y);

                ++Mark[cmp[i].x];

                cmp[i].flag=;

        }

        flag=;

        topsort();

        if(flag)

            printf("-1\n");

        else

          printf("%lld\n",sum);

    }return ;

}