#include<iostream>
#include<algorithm>
#include<stack>
#include<vector>
#include<set>

using namespace std;
using ll = long long;

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    
    int n,m;
    cin>>n>>m;
    vector<int> cnt(n,0);
    for(int i = 0;i<m;i++){
        int u,v;
        cin>>u>>v;
        u--;v--;
        cnt[u]++;
        cnt[v]--;
    }
    int sum = 0;
    int all = 0;
    for(int i = 0;i<n;i++) {
        sum += cnt[i];
        if(cnt[i]>0) all += cnt[i];
    }
    if(sum!=0) cout<<-1<<endl;
    else cout<<max(0,all-1)<<endl;
}