/* aman36 - Aman Raj */

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
 
#define IOS ios :: sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define AfterDecimal(n) cout << fixed << setprecision(n);
#define pb push_back
#define pp(a, b) pair <a, b>
#define ll long long
#define ld long double
#define all(a) a.begin(), a.end()
#define pi pair<int, int>
#define pll pair<ll, ll>
#define F first
#define S second
#define itr iterator
#define bits(x) __builtin_popcount(x)
#define fori(n)  for (ll i = 0; i < n; i++)
#define forj(n)  for (ll j = 0; j < n; j++)
#define fork(n)  for (ll k = 0; k < n; k++)
#define forlp(k,x,n)  for (ll k = x; k <= n; k++)
#define vll vector <ll>
#define mll map <ll, ll>
#define vpll vector <pll>
#define mem(x) memset(x, 0, sizeof(x));
#define int ll
// #define endl "\n"
 
using namespace __gnu_pbds;
using namespace std;

template <typename T> using ordered_set =
 tree<T, null_type, less<T>, rb_tree_tag, 
 tree_order_statistics_node_update>;
// order_of_key (k) : Number of items strictly smaller than k .
// find_by_order(k) : K-th element in a set (counting from zero).
// ordered_set <int> os;     *os.order_by_key(k);
 
#ifdef DEBUG
#define debug(...) __f(#__VA_ARGS__, __VA_ARGS__)
template<typename Arg1>
void __f(const char* name, Arg1&& arg1){
    cout << "# "<< name << " = " << arg1 << '\n';
}
template<typename Arg1, typename... Args>
void __f(const char* names, Arg1 && arg1, Args &&... args){
        const char* comma = strchr(names + 1 ,',');
        cout << "# ";
        cout.write(names, comma - names)<< " = " << arg1 << " | ";
        __f(comma + 1, args...);
}
#else
#define debug(...) 36
#endif

ll mxm() {return LLONG_MIN;}
template<typename... Args>
ll mxm(ll a, Args... args) { return max(a,mxm(args...)); }
 
ll mnm() {return LLONG_MAX;}
template<typename... Args>
ll mnm(ll a, Args... args) { return min(a,mnm(args...)); }

template<class T> ostream& operator<<(ostream& os,vector<T> V){
    os<<"[ ";for(auto v:V)os<<v<<" ";return os<<"]";
}
template<class L,class R> ostream& operator<<(ostream& os,pair<L,R> P){
    return os<<"("<<P.first<<","<<P.second<<")";
}

int power(int lx, int ly, int p) { 
    ll res = 1;
    ll x = lx;
    ll y = ly;
    x = x % p;  
    while (y > 0) {
        if (y & 1) 
            res = (res * x) % p; 
        y = y >> 1;
        x = (x * x) % p; 
    }
    return (int)res;
}

bool isPrime(int n) 
{ 
    if (n <= 1) return false; 
    if (n <= 3) return true; 
    if (n % 2 == 0 || n % 3 == 0) return false;  
    for (int i = 5; i * i <= n; i = i + 6) 
        if (n % i == 0 || n % (i + 2) == 0) 
            return false; 
    return true; 
}

const ll INF = (ll)(8e18);
const ll MOD = 1e9 + 7;
const ll MOD2 = 998244353;
// check the limits
const int maxn = 1e6+5;
const int M = 2005;

int dp[M][M];
int mod(int x, int m) {
    x %= m;
    if(x < 0) x += m;
    return x;
}

int32_t main() {
    IOS;
    #ifndef ONLINe5E_JUDGE
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    #endif

    // int tcc;
    // cin >> tcc;
    // for(int tc=1; tc<=tcc; tc++) {
    //     cout << "Case #" << tc << ": " ;
    // }

    
    int n, m;
    cin >> n >> m;

    int s=0;
    int a[m];
    fori(m) {
        cin >> a[i];
        s += a[i];
    }

    if(s > n) cout << -1 << endl;
    else cout << n-s << endl;
    return 0;

    #ifdef LOCAL
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
    #endif
}

/* g++ -DLOCAL -DDEBUG -std=c++17 -Wshadow -Wall -o "c" "icpc.cpp" 
-fsanitize=address -fsanitize=undefined -D_GLIBCXX_DEBUG -g */

// read the question correctly
// read code carefully
// corner cases // int vs ll // cin vs scanf // clear structures // maths
// check max, min set to INT_MAX instead INF
// check constraint