/*
    Created by vasu090 on 19 Apr, 2020
*/
#include <iostream>
#include <cmath>
#include <vector>
#include <algorithm>

using namespace std;

#define rep(i, a, b) for(auto (i)=a;i<b;i++)
#define list(i, N) for(auto (i)=0;i<N;i++)

typedef long long ll;
typedef vector <ll> vi;
typedef pair <ll, ll> pi;
#define mp make_pair
#define pb push_back
#define eb emplace_back

#define int ll
#define INF 1e18+5
#define mod 1000000007

template<class T>
T max(T a, T b, T c) {
    return max(a, max(b, c));
}

int32_t main() {

    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    int n,m; cin >> n >> m;
    vi a(m); list(i,m) cin >> a[i];

    int sm = 0;
    list(i,m) sm += a[i];

    if(n < sm){
        cout << "-1\n"; return 0;
    }


    cout << sm-n << '\n';
    return 0;
}