#include <bits/stdc++.h>
using namespace std;

typedef long long int ll; 
#define MOD 1000000007
#define ON 200000
#define rep(i,n) fp(i,0,n)  
#define fp(i,a,b) for(ll i=(a);i<(b);++i)
#define fn(i,a,b) for(ll i=(a);i>(b);i--)
#define MS0(X) memset((X), 0, sizeof((X)))
#define MS1(X) memset((X), -1, sizeof((X)))
#define FF first
#define SS second
typedef pair<ll, ll> pll; 
typedef vector<pll> vpll;
typedef vector<ll> vll;
#define rs(t) string t;cin>>t
#define ri(t) int t;cin>>t
#define rll(t) long long int t;cin>>t
#define MK make_pair
#define PB push_back
#define PO pop_back
template<typename T> istream &operator>>(istream &is, vector<T> &vec){ for (auto &v : vec) is >> v; return is; }
///// This part below is only for debug, not used /////
template<typename T> ostream &operator<<(ostream &os, const vector<T> &vec){ os << "["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const deque<T> &vec){ os << "deq["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa){ os << "(" << pa.first << "," << pa.second << ")"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;
///// END /////



int main(){
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  rll(n);
  rll(m);

  vll v(m);
  cin >> v;

  ll ma = 0;
  rep(i,m){
  	ma += v[i];
  }	

  if(ma>n){
  	cout << -1 << endl;
  }
  else{
  	cout << n - ma << endl;
  }
  return 0;
  
}