#include <bits/stdc++.h>

using namespace std;

#define SZ(x) (int)(x.size())

using ll = long long;
using ld = long double;
const ld eps = 1e-9;
const ll MOD = 1000000007;
const int INF = 1000000000;
const ll LINF = 1ll<<50;

template <typename T1, typename T2>
void chmax(T1 &a, const T2 &b) {
  if(a < b) a = b;
}

template <typename T1, typename T2>
void chmin(T1 &a, const T2 &b) {
  if(a > b) a = b;
}

template<typename T>
void printv(const vector<T>& s) {
  for(int i=0;i<(int)(s.size());++i) {
    cout << s[i];
    if(i == (int)(s.size())-1) cout << endl;
    else cout << " ";
  }
}

template<typename T1, typename T2>
ostream& operator<<(ostream &os, const pair<T1, T2> p) {
  os << p.first << ":" << p.second;
  return os;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int n, m; cin >> n >> m;
  vector<int> a(m);
  ll su = 0;
  for(int i=0;i<m;++i) {
    cin >> a[i];
    su += a[i];
  }
  cout << (n - su >= 0 ? n - su : -1) << endl;


  
}
