#include <iostream>
#include <algorithm>
#include <functional>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <string>
#include <numeric>
#include <cmath>

#define rep(i, n) for (int i = 0; i < n; i++)
using namespace std;
typedef long long ll;
typedef vector<int> vec;
typedef vector<vector<int>> mat;
using P = pair<ll, ll>;

template<class T> inline bool chmin(T& a,T b) {if (a > b) {a = b; return true;} return false;}
template<class T> inline bool chmax(T& a,T b) {if (a < b) {a = b; return true;} return false;}

const ll MOD = 1000000007;
const int INF = 1<<29;

int main(){
  int N, M; cin >> N >> M;
  vector<int> a(M);
  rep(i, M) cin >> a[i];
  int tmp = accumulate(a.begin(), a.end(), 0);
  if (N - tmp >= 0) cout << N - tmp << endl;
  else cout << -1 << endl;
}
