#include <iostream>

#include <algorithm>

using namespace std;

int main(){

    int n,m;

    cin >> n >> m;

    int minCorrectTime  =, maxCorrectTime = , minWrongTime = ,tmp;

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

        cin >> tmp;

        minCorrectTime = min(tmp,minCorrectTime);

        maxCorrectTime = max(tmp,maxCorrectTime);

    }

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

        cin >> tmp;

        minWrongTime = min(minWrongTime,tmp);

    }

    if(maxCorrectTime < minWrongTime && *minCorrectTime < minWrongTime) cout<<max(maxCorrectTime,*minCorrectTime)<<endl;

    else cout<<-<<endl;

    return ;

}