Problem p02706 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define fo(a, b) for (int a = 0; a < b; a++)
#define Sort(a) sort(a.begin(), a.end())
#define rev(a) reverse(a.begin(), a.end())
#define fi first
#define se second
#define sz size()
#define bgn begin()
#define en end()
#define pb push_back
#define pp() pop_back()
#define V vector
#define P pair
#define yuko(a) setprecision(a)
#define uni(a) a.erase(unique(a.begin(), a.end()), a.end())
#define Q queue
#define pri priority_queue
#define Pri priority_queue<int, vector<int>, greater<int>>
#define PriP                                                                   \
  priority_queue<P<int, int>, vector<P<int, int>>, greater<P<int, int>>>
#define ff first.first
#define fs first.second
#define sf second.first
#define ss second.second
#define all(a) (a).begin(), (a).end()
#define elif else if
int low(V<int> &a, int b) {
  auto c = lower_bound(a.begin(), a.end(), b);
  int d = c - a.bgn;
  return d;
}
int upp(V<int> &a, int b) {
  auto c = upper_bound(a.begin(), a.end(), b);
  int d = c - a.bgn;
  return d;
}
template <class T> void cou(vector<vector<T>> a) {
  int b = a.size();
  int c = a[0].size();
  fo(i, b) {
    fo(j, c) {
      cout << a[i][j];
      if (j == c - 1)
        cout << endl;
      else
        cout << ' ';
    }
  }
}
int wari(int a, int b) {
  if (a % b == 0)
    return a / b;
  else
    return a / b + 1;
}
int keta(int a) {
  double b = a;
  b = log10(b);
  int c = b;
  return c + 1;
}
int souwa(int a) { return a * (a + 1) / 2; }
int gcm(int a, int b) {
  if (a % b == 0)
    return b;
  return gcm(b, a % b);
}
bool prime(int a) {
  if (a < 2)
    return false;
  else if (a == 2)
    return true;
  else if (a % 2 == 0)
    return false;
  for (int i = 3; i <= sqrt(a) + 1; i += 2) {
    if (a % i == 0)
      return false;
  }
  return true;
}
struct Union {
  vector<int> par;
  Union(int a) { par = vector<int>(a, -1); }
  int find(int a) {
    if (par[a] < 0)
      return a;
    else
      return par[a] = find(par[a]);
  }
  bool same(int a, int b) { return find(a) == find(b); }
  int Size(int a) { return -par[find(a)]; }
  void unite(int a, int b) {
    a = find(a);
    b = find(b);
    if (a == b)
      return;
    if (Size(b) > Size(a))
      swap<int>(a, b);
    par[a] += par[b];
    par[b] = a;
  }
};
int ketas(int a) {
  string b = to_string(a);
  int c = 0;
  fo(i, keta(a)) { c += b[i] - '0'; }
  return c;
}
bool fe(int a, int b) {
  a %= 10;
  b %= 10;
  if (a == 0)
    a = 10;
  if (b == 0)
    b = 10;
  if (a > b)
    return true;
  else
    return false;
}
int INF = 1000000007;
struct edge {
  int s, t, d;
};
V<int> mojisyu(string a) {
  V<int> b(26, 0);
  fo(i, a.sz) { b[a[i] - 'a']++; }
  return b;
}
int wa2(int a) {
  if (a % 2 == 1)
    return a / 2;
  return a / 2 - 1;
}
/*signed main(){
  int a,b,c;
  cin>>a>>b>>c;
  V<V<edge>> d(a);
  fo(i,b){
    edge e;
    cin>>e.s>>e.t>>e.d;
    d[e.s].pb(e);
  }
  V<int> e(a,INF);
  e[c]=0;
  priority_queue<P<int,int>,V<P<int,int>>,greater<P<int,int>>> f;
  f.push({0,c});
  int h=INF;
  while(!f.empty()){
    P<int,int> g;
    g=f.top();
    f.pop();
    int v = g.second, i = g.first;
    for(edge l : d[v]){
      if(e[l.t] > i + l.d){
        e[l.t] = i + l.d;
        f.push({i+l.d , l.t});
      }
    }
  }
  fo(i,a){
    if(e[i]==INF)
      cout<<"INF"<<endl;
    else
      cout<<e[i]<<endl;
  }
}
?*/
int nCr(int n, int r) {
  int a = 1;
  r = min(r, n - r);
  for (int i = n; i > n - r; i--) {
    a *= i;
    a /= n - i + 1;
  }
  return a;
}
/*void sea(int x,int y){
  if(x<0||a<=x||y<0||b<=y||c[x][y]=='#')
    return;
  if(d[x][y])
    return;
  d[x][y]++;
  sea(x+1,y);
  sea(x-1,y);
  sea(x,y+1);
  sea(x,y-1);
}*/
int kaijou(int a) {
  int b = 1;
  fo(i, a) b *= i + 1;
  return b;
}
int nPr(int a, int b) {
  if (a < b)
    return 0;
  if (b == 0)
    return 1;
  int c = 1;
  for (int i = a; i > a - b; i--) {
    c *= i;
    c %= INF;
  }
  return c;
}
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

// a^{-1} mod を計算する
long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }

int lcm(int a, int b) {
  int c = modinv(gcm(a, b), INF);
  return ((a * c) % INF) * (b % INF) % INF;
}
int MOD = INF;
int fac[1000010], finv[1000010], inv[1000010];
// テーブルを作る前処理
// 先にCOMinit()で前処理をする
// ABC145D
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < 1000010; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
// 二項係数計算
int COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
bool naka(int a, int b, V<V<char>> c) {
  return (a >= 0 && b >= 0 && a < c.sz && b < c[0].sz);
}

V<P<int, int>> mawari8 = {{0, -1},  {0, 1}, {1, 0},  {-1, 0},
                          {-1, -1}, {1, 1}, {1, -1}, {-1, -1}};
int inf = 1000000000000000007;

V<P<int, int>> mawari4 = {{0, -1}, {0, 1}, {1, 0}, {-1, 0}};
// 最短経路の表 a(全部INFで初期化)
// 縦横 x,y
// 迷路 f
// スタートsx,sy
// ゴールgx,gy
// 文字はgから使おうね
/*int bfs_haba(){
  Q<P<int,int>> b;
  a[sx][sy]=0;
  b.push({sx,sy});
  while(!b.empty()){
    P<int,int> c=b.front();
    b.pop();
    if(c.fi==gx&&c.se==gy){
      break;
    }
    fo(i,4){
      int d=c.fi+mawari4[i].fi;
      int e=c.se+mawari4[i].se;
      if(0<=d&&0<=e&&d<x&&e<y&&f[d][e]!='#'&&a[d][e]==INF){
        b.push({d,e});
        a[d][e]=1+a[c.fi][c.se];
      }
    }
  }
  return a[gx][gy];
}*/
V<int> onajibubun(string a) {
  V<int> b(a.sz);
  for (int i = 1, j = 0; i < a.sz; i++) {
    if (i + b[i - j] < j + b[j])
      b[i] = b[i - j];
    else {
      int c = max<int>(0, j + b[j] - i);
      while (i + c < a.sz && a[c] == a[i + c])
        c++;
      b[i] = c;
      j = i;
    }
  }
  b[0] = a.sz;
  return b;
}
// 各頂点ごとにどこに辺が出てるかの表がc
// 各頂点ごとの色を表すV<int>(頂点数max)のcolorを用意する
// aはどこ塗るか、bは何で塗るかなので、(0,1,c)でよぶとおけ
V<int> color(200005);
bool nibu_hantei(int a, int b, V<V<int>> c) {
  color[a] = b;
  fo(i, c[a].sz) {
    if (b == color[c[a][i]])
      return false;
    if (color[c[a][i]] == 0 && !nibu_hantei(c[a][i], -b, c))
      return false;
  }
  return true;
}
// aは頂点数
// nibu_hanteiの上にcolorを用意する
// 各頂点ごとにどこに辺が出てるかの表がc
bool renketujanai_nibu_hantei(int a, V<V<int>> c) {
  fo(i, a) {
    if (color[i] == 0) {
      if (!nibu_hantei(i, 1, c))
        return false;
    }
  }
  return true;
}
struct segmin {
  vector<int> seg;
  int b;
  segmin(V<int> a) {
    b = 1;
    while (b < a.sz)
      b *= 2;
    seg = vector<int>(2 * b - 1, inf);
    fo(i, a.sz) { seg[i + b - 1] = a[i]; }
    for (int i = b - 2; i >= 0; i--) {
      seg[i] = min(seg[2 * i + 1], seg[2 * i + 2]);
    }
  }
  void update(int i, int a) {
    i += b - 1;
    seg[i] = a;
    while (i) {
      i = (i - 1) / 2;
      seg[i] = min(seg[2 * i + 1], seg[2 * i + 2]);
    }
  }
  // 最初呼び出すときは要求区間x,y+1(yは添え字+1)とa=0,l=0,r=INFで
  // l,rは探すところ
  int getmin(int x, int y, int a, int l, int r) {
    if (r == INF)
      r = b;
    if (r <= x || y <= l)
      return INF;
    if (x <= l && r <= y)
      return seg[a];
    int a1 = getmin(x, y, 2 * a + 1, l, (l + r) / 2);
    int a2 = getmin(x, y, 2 * a + 2, (l + r) / 2, r);
    return min(a1, a2);
  }
};
struct segadd {
  vector<int> seg;
  int b;
  segadd(V<int> a) {
    b = 1;
    while (b < a.sz)
      b *= 2;
    seg = vector<int>(2 * b - 1, 0);
    fo(i, a.sz) { seg[i + b - 1] = a[i]; }
    for (int i = b - 2; i >= 0; i--) {
      seg[i] = seg[2 * i + 1] + seg[2 * i + 2];
    }
  }
  void update(int i, int a) {
    i += b - 1;
    seg[i] = a;
    while (i) {
      i = (i - 1) / 2;
      seg[i] = seg[2 * i + 1] + seg[2 * i + 2];
    }
  }
  // 最初呼び出すときは要求区間x,y+1(yは添え字+1)とa=0,l=0,r=INFで
  // l,rは探すところ
  int getadd(int x, int y, int a, int l, int r) {
    if (r == INF)
      r = b;
    if (r <= x || y <= l)
      return 0;
    if (x <= l && r <= y)
      return seg[a];
    int a1 = getadd(x, y, 2 * a + 1, l, (l + r) / 2);
    int a2 = getadd(x, y, 2 * a + 2, (l + r) / 2, r);
    return a1 + a2;
  }
};
struct sege {
  vector<P<int, V<int>>> seg;
  int b;
  sege(string a) {
    b = 1;
    while (b < a.sz)
      b *= 2;
    seg = vector<P<int, V<int>>>(2 * b - 1);
    fo(i, a.sz) {
      seg[i + b - 1].fi = 1;
      seg[i + b - 1].se.pb(a[i] - 'a');
    }
    for (int i = b - 2; i >= 0; i--) {
      V<int> d = seg[2 * i + 1].se;
      fo(j, seg[2 * i + 2].se.sz) { d.pb(seg[2 * i + 2].se[j]); }
      Sort(d);
      uni(d);
      seg[i].se = d;
      seg[i].fi = d.sz;
    }
  }
  V<int> mu;
  void update(int i, char a) {
    i += b - 1;
    seg[i].se = mu;
    seg[i].se.pb(a - 'a');
    seg[i].fi = 1;
    while (i) {
      i = (i - 1) / 2;
      V<int> d = seg[2 * i + 1].se;
      fo(j, seg[2 * i + 2].se.sz) { d.pb(seg[2 * i + 2].se[j]); }
      Sort(d);
      uni(d);
      seg[i].se = d;
      seg[i].fi = d.sz;
    }
  }
  void unko() { fo(i, 2 * b - 1) cout << seg[i].fi << ' '; }
  // 最初呼び出すときは要求区間x,y+1(yは添え字+1)とa=0,l=0,r=INFで
  // l,rは探すところ
  P<int, V<int>> gete(int x, int y, int a, int l, int r) {
    if (r == INF)
      r = b;
    if (r <= x || y <= l)
      return {0, mu};
    if (x <= l && r <= y)
      return seg[a];
    P<int, V<int>> a1 = gete(x, y, 2 * a + 1, l, (l + r) / 2);
    P<int, V<int>> a2 = gete(x, y, 2 * a + 2, (l + r) / 2, r);
    fo(i, a2.se.sz) a1.se.pb(a2.se[i]);
    Sort(a1.se);
    uni(a1.se);
    return {a1.se.sz, a1.se};
  }
}; /*
 signed main(){
   int a,b,c;
   cin>>a>>b>>c;
   V<V<edge>> d(a);
   fo(i,b){
     edge e;
     cin>>e.s>>e.t>>e.d;
     d[e.s].pb(e);
   }
   V<int> e(a,INF);
   e[c]=0;
   priority_queue<P<int,int>,V<P<int,int>>,greater<P<int,int>>> f;
   f.push({0,c});
   int h=INF;
   while(!f.empty()){
     P<int,int> g;
     g=f.top();
     f.pop();
     int v = g.second, i = g.first;
     for(edge l : d[v]){
       if(e[l.t] > i + l.d){
         e[l.t] = i + l.d;
         f.push({i+l.d , l.t});
       }
     }
   }
   fo(i,a){
     if(e[i]==INF)
       cout<<"INF"<<endl;
     else
       cout<<e[i]<<endl;
   }
 }*/
// aのb乗mod c
int kurikaesijijou(int a, int b, int c) {
  a %= c;
  b %= c;
  if (b == 0)
    return 1;
  if (b % 2 == 0) {
    int d = kurikaesijijou(a, b / 2, c);
    d %= c;
    return d * d % c;
  }
  return a * kurikaesijijou(a, b - 1, c) % c;
}
signed main() {
  int a, b;
  cin >> a >> b;
  V<int> c(b);
  fo(i, b) cin >> c[i];
  int d = 0;
  fo(i, a) d += c[i];
  cout << max<int>(-1, a - d) << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define fo(a, b) for (int a = 0; a < b; a++)
#define Sort(a) sort(a.begin(), a.end())
#define rev(a) reverse(a.begin(), a.end())
#define fi first
#define se second
#define sz size()
#define bgn begin()
#define en end()
#define pb push_back
#define pp() pop_back()
#define V vector
#define P pair
#define yuko(a) setprecision(a)
#define uni(a) a.erase(unique(a.begin(), a.end()), a.end())
#define Q queue
#define pri priority_queue
#define Pri priority_queue<int, vector<int>, greater<int>>
#define PriP                                                                   \
  priority_queue<P<int, int>, vector<P<int, int>>, greater<P<int, int>>>
#define ff first.first
#define fs first.second
#define sf second.first
#define ss second.second
#define all(a) (a).begin(), (a).end()
#define elif else if
int low(V<int> &a, int b) {
  auto c = lower_bound(a.begin(), a.end(), b);
  int d = c - a.bgn;
  return d;
}
int upp(V<int> &a, int b) {
  auto c = upper_bound(a.begin(), a.end(), b);
  int d = c - a.bgn;
  return d;
}
template <class T> void cou(vector<vector<T>> a) {
  int b = a.size();
  int c = a[0].size();
  fo(i, b) {
    fo(j, c) {
      cout << a[i][j];
      if (j == c - 1)
        cout << endl;
      else
        cout << ' ';
    }
  }
}
int wari(int a, int b) {
  if (a % b == 0)
    return a / b;
  else
    return a / b + 1;
}
int keta(int a) {
  double b = a;
  b = log10(b);
  int c = b;
  return c + 1;
}
int souwa(int a) { return a * (a + 1) / 2; }
int gcm(int a, int b) {
  if (a % b == 0)
    return b;
  return gcm(b, a % b);
}
bool prime(int a) {
  if (a < 2)
    return false;
  else if (a == 2)
    return true;
  else if (a % 2 == 0)
    return false;
  for (int i = 3; i <= sqrt(a) + 1; i += 2) {
    if (a % i == 0)
      return false;
  }
  return true;
}
struct Union {
  vector<int> par;
  Union(int a) { par = vector<int>(a, -1); }
  int find(int a) {
    if (par[a] < 0)
      return a;
    else
      return par[a] = find(par[a]);
  }
  bool same(int a, int b) { return find(a) == find(b); }
  int Size(int a) { return -par[find(a)]; }
  void unite(int a, int b) {
    a = find(a);
    b = find(b);
    if (a == b)
      return;
    if (Size(b) > Size(a))
      swap<int>(a, b);
    par[a] += par[b];
    par[b] = a;
  }
};
int ketas(int a) {
  string b = to_string(a);
  int c = 0;
  fo(i, keta(a)) { c += b[i] - '0'; }
  return c;
}
bool fe(int a, int b) {
  a %= 10;
  b %= 10;
  if (a == 0)
    a = 10;
  if (b == 0)
    b = 10;
  if (a > b)
    return true;
  else
    return false;
}
int INF = 1000000007;
struct edge {
  int s, t, d;
};
V<int> mojisyu(string a) {
  V<int> b(26, 0);
  fo(i, a.sz) { b[a[i] - 'a']++; }
  return b;
}
int wa2(int a) {
  if (a % 2 == 1)
    return a / 2;
  return a / 2 - 1;
}
/*signed main(){
  int a,b,c;
  cin>>a>>b>>c;
  V<V<edge>> d(a);
  fo(i,b){
    edge e;
    cin>>e.s>>e.t>>e.d;
    d[e.s].pb(e);
  }
  V<int> e(a,INF);
  e[c]=0;
  priority_queue<P<int,int>,V<P<int,int>>,greater<P<int,int>>> f;
  f.push({0,c});
  int h=INF;
  while(!f.empty()){
    P<int,int> g;
    g=f.top();
    f.pop();
    int v = g.second, i = g.first;
    for(edge l : d[v]){
      if(e[l.t] > i + l.d){
        e[l.t] = i + l.d;
        f.push({i+l.d , l.t});
      }
    }
  }
  fo(i,a){
    if(e[i]==INF)
      cout<<"INF"<<endl;
    else
      cout<<e[i]<<endl;
  }
}
?*/
int nCr(int n, int r) {
  int a = 1;
  r = min(r, n - r);
  for (int i = n; i > n - r; i--) {
    a *= i;
    a /= n - i + 1;
  }
  return a;
}
/*void sea(int x,int y){
  if(x<0||a<=x||y<0||b<=y||c[x][y]=='#')
    return;
  if(d[x][y])
    return;
  d[x][y]++;
  sea(x+1,y);
  sea(x-1,y);
  sea(x,y+1);
  sea(x,y-1);
}*/
int kaijou(int a) {
  int b = 1;
  fo(i, a) b *= i + 1;
  return b;
}
int nPr(int a, int b) {
  if (a < b)
    return 0;
  if (b == 0)
    return 1;
  int c = 1;
  for (int i = a; i > a - b; i--) {
    c *= i;
    c %= INF;
  }
  return c;
}
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

// a^{-1} mod を計算する
long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }

int lcm(int a, int b) {
  int c = modinv(gcm(a, b), INF);
  return ((a * c) % INF) * (b % INF) % INF;
}
int MOD = INF;
int fac[1000010], finv[1000010], inv[1000010];
// テーブルを作る前処理
// 先にCOMinit()で前処理をする
// ABC145D
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < 1000010; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
// 二項係数計算
int COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
bool naka(int a, int b, V<V<char>> c) {
  return (a >= 0 && b >= 0 && a < c.sz && b < c[0].sz);
}

V<P<int, int>> mawari8 = {{0, -1},  {0, 1}, {1, 0},  {-1, 0},
                          {-1, -1}, {1, 1}, {1, -1}, {-1, -1}};
int inf = 1000000000000000007;

V<P<int, int>> mawari4 = {{0, -1}, {0, 1}, {1, 0}, {-1, 0}};
// 最短経路の表 a(全部INFで初期化)
// 縦横 x,y
// 迷路 f
// スタートsx,sy
// ゴールgx,gy
// 文字はgから使おうね
/*int bfs_haba(){
  Q<P<int,int>> b;
  a[sx][sy]=0;
  b.push({sx,sy});
  while(!b.empty()){
    P<int,int> c=b.front();
    b.pop();
    if(c.fi==gx&&c.se==gy){
      break;
    }
    fo(i,4){
      int d=c.fi+mawari4[i].fi;
      int e=c.se+mawari4[i].se;
      if(0<=d&&0<=e&&d<x&&e<y&&f[d][e]!='#'&&a[d][e]==INF){
        b.push({d,e});
        a[d][e]=1+a[c.fi][c.se];
      }
    }
  }
  return a[gx][gy];
}*/
V<int> onajibubun(string a) {
  V<int> b(a.sz);
  for (int i = 1, j = 0; i < a.sz; i++) {
    if (i + b[i - j] < j + b[j])
      b[i] = b[i - j];
    else {
      int c = max<int>(0, j + b[j] - i);
      while (i + c < a.sz && a[c] == a[i + c])
        c++;
      b[i] = c;
      j = i;
    }
  }
  b[0] = a.sz;
  return b;
}
// 各頂点ごとにどこに辺が出てるかの表がc
// 各頂点ごとの色を表すV<int>(頂点数max)のcolorを用意する
// aはどこ塗るか、bは何で塗るかなので、(0,1,c)でよぶとおけ
V<int> color(200005);
bool nibu_hantei(int a, int b, V<V<int>> c) {
  color[a] = b;
  fo(i, c[a].sz) {
    if (b == color[c[a][i]])
      return false;
    if (color[c[a][i]] == 0 && !nibu_hantei(c[a][i], -b, c))
      return false;
  }
  return true;
}
// aは頂点数
// nibu_hanteiの上にcolorを用意する
// 各頂点ごとにどこに辺が出てるかの表がc
bool renketujanai_nibu_hantei(int a, V<V<int>> c) {
  fo(i, a) {
    if (color[i] == 0) {
      if (!nibu_hantei(i, 1, c))
        return false;
    }
  }
  return true;
}
struct segmin {
  vector<int> seg;
  int b;
  segmin(V<int> a) {
    b = 1;
    while (b < a.sz)
      b *= 2;
    seg = vector<int>(2 * b - 1, inf);
    fo(i, a.sz) { seg[i + b - 1] = a[i]; }
    for (int i = b - 2; i >= 0; i--) {
      seg[i] = min(seg[2 * i + 1], seg[2 * i + 2]);
    }
  }
  void update(int i, int a) {
    i += b - 1;
    seg[i] = a;
    while (i) {
      i = (i - 1) / 2;
      seg[i] = min(seg[2 * i + 1], seg[2 * i + 2]);
    }
  }
  // 最初呼び出すときは要求区間x,y+1(yは添え字+1)とa=0,l=0,r=INFで
  // l,rは探すところ
  int getmin(int x, int y, int a, int l, int r) {
    if (r == INF)
      r = b;
    if (r <= x || y <= l)
      return INF;
    if (x <= l && r <= y)
      return seg[a];
    int a1 = getmin(x, y, 2 * a + 1, l, (l + r) / 2);
    int a2 = getmin(x, y, 2 * a + 2, (l + r) / 2, r);
    return min(a1, a2);
  }
};
struct segadd {
  vector<int> seg;
  int b;
  segadd(V<int> a) {
    b = 1;
    while (b < a.sz)
      b *= 2;
    seg = vector<int>(2 * b - 1, 0);
    fo(i, a.sz) { seg[i + b - 1] = a[i]; }
    for (int i = b - 2; i >= 0; i--) {
      seg[i] = seg[2 * i + 1] + seg[2 * i + 2];
    }
  }
  void update(int i, int a) {
    i += b - 1;
    seg[i] = a;
    while (i) {
      i = (i - 1) / 2;
      seg[i] = seg[2 * i + 1] + seg[2 * i + 2];
    }
  }
  // 最初呼び出すときは要求区間x,y+1(yは添え字+1)とa=0,l=0,r=INFで
  // l,rは探すところ
  int getadd(int x, int y, int a, int l, int r) {
    if (r == INF)
      r = b;
    if (r <= x || y <= l)
      return 0;
    if (x <= l && r <= y)
      return seg[a];
    int a1 = getadd(x, y, 2 * a + 1, l, (l + r) / 2);
    int a2 = getadd(x, y, 2 * a + 2, (l + r) / 2, r);
    return a1 + a2;
  }
};
struct sege {
  vector<P<int, V<int>>> seg;
  int b;
  sege(string a) {
    b = 1;
    while (b < a.sz)
      b *= 2;
    seg = vector<P<int, V<int>>>(2 * b - 1);
    fo(i, a.sz) {
      seg[i + b - 1].fi = 1;
      seg[i + b - 1].se.pb(a[i] - 'a');
    }
    for (int i = b - 2; i >= 0; i--) {
      V<int> d = seg[2 * i + 1].se;
      fo(j, seg[2 * i + 2].se.sz) { d.pb(seg[2 * i + 2].se[j]); }
      Sort(d);
      uni(d);
      seg[i].se = d;
      seg[i].fi = d.sz;
    }
  }
  V<int> mu;
  void update(int i, char a) {
    i += b - 1;
    seg[i].se = mu;
    seg[i].se.pb(a - 'a');
    seg[i].fi = 1;
    while (i) {
      i = (i - 1) / 2;
      V<int> d = seg[2 * i + 1].se;
      fo(j, seg[2 * i + 2].se.sz) { d.pb(seg[2 * i + 2].se[j]); }
      Sort(d);
      uni(d);
      seg[i].se = d;
      seg[i].fi = d.sz;
    }
  }
  void unko() { fo(i, 2 * b - 1) cout << seg[i].fi << ' '; }
  // 最初呼び出すときは要求区間x,y+1(yは添え字+1)とa=0,l=0,r=INFで
  // l,rは探すところ
  P<int, V<int>> gete(int x, int y, int a, int l, int r) {
    if (r == INF)
      r = b;
    if (r <= x || y <= l)
      return {0, mu};
    if (x <= l && r <= y)
      return seg[a];
    P<int, V<int>> a1 = gete(x, y, 2 * a + 1, l, (l + r) / 2);
    P<int, V<int>> a2 = gete(x, y, 2 * a + 2, (l + r) / 2, r);
    fo(i, a2.se.sz) a1.se.pb(a2.se[i]);
    Sort(a1.se);
    uni(a1.se);
    return {a1.se.sz, a1.se};
  }
}; /*
 signed main(){
   int a,b,c;
   cin>>a>>b>>c;
   V<V<edge>> d(a);
   fo(i,b){
     edge e;
     cin>>e.s>>e.t>>e.d;
     d[e.s].pb(e);
   }
   V<int> e(a,INF);
   e[c]=0;
   priority_queue<P<int,int>,V<P<int,int>>,greater<P<int,int>>> f;
   f.push({0,c});
   int h=INF;
   while(!f.empty()){
     P<int,int> g;
     g=f.top();
     f.pop();
     int v = g.second, i = g.first;
     for(edge l : d[v]){
       if(e[l.t] > i + l.d){
         e[l.t] = i + l.d;
         f.push({i+l.d , l.t});
       }
     }
   }
   fo(i,a){
     if(e[i]==INF)
       cout<<"INF"<<endl;
     else
       cout<<e[i]<<endl;
   }
 }*/
// aのb乗mod c
int kurikaesijijou(int a, int b, int c) {
  a %= c;
  b %= c;
  if (b == 0)
    return 1;
  if (b % 2 == 0) {
    int d = kurikaesijijou(a, b / 2, c);
    d %= c;
    return d * d % c;
  }
  return a * kurikaesijijou(a, b - 1, c) % c;
}
signed main() {
  int a, b;
  cin >> a >> b;
  V<int> c(b);
  fo(i, b) cin >> c[i];
  int d = 0;
  fo(i, b) d += c[i];
  cout << max<int>(-1, a - d) << endl;
}