/*
 * Validator
 */
#include<bits/stdc++.h>
int read_int(int min, int max, char end){
    char c;
    int ans=0;
    while(true){
        c=getchar();
        if(c<'0'||'9'<c){
            assert(c==end);
            break;
        }
        ans*=10;
        ans+=c-'0';
    }
    assert(min<=ans&&ans<=max);
    return ans;
}
int main(){
    read_int(2,300,' ');
    int m=read_int(1,300,' ');
    read_int(0,300,'\n');
    std::vector<std::pair<int,int>>pq(m);
    for(int i=0;i<m;i++){
        int p=read_int(1,300,' ');
        int q=read_int(1,300,' ');
        read_int(0,300,'\n');
        pq.at(i)={p,q};
        assert(p!=q);
    }
    std::sort(pq.begin(),pq.end());
    for(int i=0;i<m-1;i++){
        auto[p0,q0]=pq.at(i);
        auto[p1,q1]=pq.at(i+1);
        assert(p0!=p1||q0!=q1);
    }
    char c=getchar();
    assert(c==EOF);
    std::cout<<-1<<std::endl;
}