/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author zjsdut
 */

#ifndef ALGO_UTILS
#define ALGO_UTILS
#include <algorithm>
#include <vector>
template<typename A, typename B>
bool chkmin(A &a, const B &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

template<typename A, typename B>
bool chkmax(A &a, const B &b) {
  if (b > a) {
    a = b;
    return true;
  }
  return false;
}

using ll = long long;
using ull = unsigned long long;
using vl = std::vector<ll>;
using vb = std::vector<bool>;
using vi = std::vector<int>;
using pii = std::pair<int, int>;
using pli = std::pair<ll, int>;
using pll = std::pair<ll, ll>;
template<typename T> using vv = std::vector<std::vector<T>>;
#define rng(i, a, b) for (std::common_type<decltype(a),decltype(b)>::type i = a; i < b; ++i)
#define up(i, a, b) for (std::common_type<decltype(a),decltype(b)>::type  i = a; i <= b; ++i)
#define down(i, b, a) for (std::common_type<decltype(a),decltype(b)>::type i = b; i >= a; i--)
#define TOKENPASTE(x, y) x##y
#define TOKENPASTE2(x, y) TOKENPASTE(x, y)
#define rep(n)                                                                 \
  for (int TOKENPASTE2(_iter_, __LINE__) = 0,                                  \
                               TOKENPASTE2(_num_, __LINE__) = n;        \
       TOKENPASTE2(_iter_, __LINE__) < TOKENPASTE2(_num_, __LINE__);           \
       ++TOKENPASTE2(_iter_, __LINE__))
#define FOR(x, cont) for (const auto &x : cont)
#define For(x, cont) for (auto &x : cont)
#define all(x) begin(x), end(x)
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define ep emplace
#define SZ(x) (int)(x).size()
#define UNIQ(vec) (vec).erase(std::unique(all(vec)), std::end(vec))
#define LB(cont, x) int(std::lower_bound(all(cont), x) - std::begin(cont))
#define UB(cont, x) int(std::upper_bound(all(cont), x) - std::begin(cont))
#endif
#ifndef JHELPER_EXAMPLE_PROJECT_LIBRARY_IO_HPP_
#define JHELPER_EXAMPLE_PROJECT_LIBRARY_IO_HPP_
#include <iostream>
#include <iomanip>
struct fast_ios {
  fast_ios() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);
    std::cout << std::fixed << std::setprecision(10);
  };
} fast_ios_;

template<typename T>
std::istream &operator>>(std::istream &stream, std::vector<T> &vec) {
  for (auto &x : vec)
    stream >> x;
  return stream;
}

template<typename T, typename U>
std::istream &operator>>(std::istream &in, std::pair<T, U> &p) {
  in >> p.first >> p.second;
  return in;
}

void scan() {}

template<class T, class... Args>
void scan(T &a, Args &... rest) {
  std::cin >> a;
  scan(rest...);
}

template<typename T>
std::ostream &operator<<(std::ostream &stream, const std::vector<T> &vec) {
  bool first = true;
  for (const T &t : vec) {
    if (first) first = false;
    else std::cout << ' ';
    std::cout << t;
  }
  return stream;
}

template<typename T>
void print(const std::vector<T> &t) {
  std::cout << t << '\n';
}

template<typename T>
void print(const std::vector<std::vector<T>> &t) {
  for (const auto &row : t) {
    print(row);
  }
}

template<typename T>
void print(const T &t) { std::cout << t << ' '; }

template<typename T, typename... Args>
void print(const T &t, const Args &... rest) {
  print(t);
  print(rest...);
}

template<typename T>
void println(const T &t) { std::cout << t << '\n'; }

template<typename T, typename... Args>
void println(const T &t, const Args &... rest) {
  print(t);
  println(rest...);
}
#endif //JHELPER_EXAMPLE_PROJECT_LIBRARY_IO_HPP_

#include <numeric>
using namespace std;

class BHomework {
  static void SOLVE() {
    int n, m;
    scan(n, m);
    vi a(m); scan(a);
    int tot = accumulate(all(a), 0);
    println(tot > n ? -1 : n - tot);
  }

public:
  static void solve(std::istream &in, std::ostream &out) {
    auto cin_buff = std::cin.rdbuf(in.rdbuf());
    auto cerr_buff = std::cerr.rdbuf(std::cout.rdbuf(out.rdbuf()));
    SOLVE();
    std::cout.rdbuf(std::cerr.rdbuf(cerr_buff));
    std::cin.rdbuf(cin_buff);
  }
};


int main() {
  BHomework solver;
  std::istream& in(std::cin);
  std::ostream& out(std::cout);
  solver.solve(in, out);
  return 0;
}


Measured memory_bytes: 3660