#include <vector>
#include <set>
#include <map>
#include <list>
#include <string>
#include <cmath>
#include <ctime>
#include <stdio.h>
#include <iostream>
#include <algorithm>

using namespace std;


#define dd long double
#define ll long long
#define PI acos(-1)
#define mp make_pair
#define pb push_back

#define INF int(1e9+7)
#define EPS 1e-12
#define sqr(a) (a)*(a)


vector <ll> a;
ll n, d, ii = 0, jj = 1, m;

bool cmp(ll x, ll y){
    return (x < y);
}


bool bc(ll checking){



    if (1 >= checking){
        return false;
    }
    int l = 0, r = n-3;

    while(l<=r){
        m = (l+r)/2;
        if (a[m] < checking && a[m+1] > checking) return true;
        if (a[m] < checking) l = m+1;
        else
            if (a[m] > checking) r = m;
        else
            return false;

        if (a[m] == checking) return false;

        if (l == 0 && r == 1 && a[0] != checking && a[1] != checking)
            return true;
    }
    return true;
}


int main(){

    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);


    cin >> d >> n;

    a.resize(n-2);

    for (int z = 0; z < n-2; ++z){
        cin >> a[z];
        d -= a[z];
    }

    if (d <= 2){
        cout << -1;
        return 0;
    }

    sort(a.begin(), a.end(), cmp);



    while(ii < n-2){
        while(jj < a[ii]){
            if (bc(d-jj) && (jj < d-jj)){
                cout << jj;
                return 0;
            }
            ++jj;
        }
        ++jj;
        ++ii;
    }

    if (bc(d-jj)){
        cout << jj;
        return 0;
    }

    cout << -1;


    return 0;
}