#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[20010];

void topsort(){
    int l=0;
    while(1){
        int j;
        int s=0;
        for(j=1;j<=n;j++){
            if(Mark[j]==0){
                sum+=num;
                --Mark[j];
                ++s;
                vis[s]=j;
                ++l;
            }
        }
        num++;
        if(l==n)
           break;
        if(!s){
            flag=1;
            break;
        }
        else{      
            for(int i=1;i<=s;i++){
            for(int k=1;k<=m;k++){
                if(cmp[k].y==vis[i]&&Mark[cmp[k].x]!=0&&cmp[k].flag==0){
                    --Mark[cmp[k].x];
                    cmp[k].flag=1;
                }
            }
            }
        }
    }
}

int main()
{

    while(scanf("%d %d",&n,&m)!=EOF){
        sum=0;
        num=888;
        memset(vis,0,sizeof(vis));
        memset(Mark,0,sizeof(Mark));
        for(int i=1;i<=m;i++){
            scanf("%d %d",&cmp[i].x,&cmp[i].y);
                ++Mark[cmp[i].x];
                cmp[i].flag=0;
        }
        flag=0;
        topsort();
        if(flag)
            printf("-1\n");
        else
          printf("%lld\n",sum);
    }return 0;
}