#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define REP(i,m,n) for(int i=(int)(m); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define RREP(i,m,n) for(int i=(int)(m); i>=(int)(n); i--)
#define rrep(i,n) RREP(i,n-1,0)
#define REPL(i,m,n) for(ll i=(ll)(m); i<(ll)(n); i++)
#define repl(i,n) REPL(i,0,n)
#define all(v) v.begin(), v.end()
const int inf = 1e9+7;
const ll longinf = 1LL<<60;
const ll mod = 1e9+7;
int main() {
  int N, M;
  cin >> N >> M;
  vector<ll> A(M);
  rep(i, M) cin >> A[i];
  ll sum = accumulate(all(A), 0LL);
  if(N-sum < 0) cout << -1 << endl;
  else cout << N-sum << endl;
  return 0;
}
