#include <bits/stdc++.h>

#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORTASC(c) sort((c).begin(),(c).end())
#define SORTDESC(c,t) sort((c).begin(),(c).end(), std::greater<t>());
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define VMAX(a) *max_element(ALL(a))
#define VMIN(a) *min_element(ALL(a))
#define OUT(a) cout << (a) << endl;
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

const double PI  = acos(-1.0);

template <class T, class S>
using umap = std::unordered_map<T, S>;
using ll = long long;
using namespace std;

int main() {
    ll n, m, t = 0;
    ll a[200010];

    cin >> n >> m;

    REP(i, m){
      ll tmp;
      cin >> tmp;
      t += tmp;
    }

    if (t > n){
      cout << -1 << endl;
    } else {
      cout << n-t << endl;
    }

    return 0;
}