#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++ (i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++ (i))
#define REP_R(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define REP3R(i, m, n) for (int i = (int)(n) - 1; (i) >= (int)(m); -- (i))
#define FOR(i, m, n) for(int i = m; i < n; i++)
#define FORR(i, a, b) for(int i=(b)-1; i>=(a); i--)
#define ALL(v) v.begin(), v.end()
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(),(c).end())
#define SORTR(c) sort((c).begin(),(c).end(),greater<long long>());
#define BIT(n) (1LL<<(n))
#define PCNT(x) __builtin_popcountll(x)
#define GCD(x,y) __gcd(x,y)
#define LCM(x,y) x/__gcd(x,y)*y
#define PB push_back
#define MP make_pair
#define endl "\n"
#define pa cout << ans << endl
#define p(x) cout << x << endl
#define couta(x) for(auto iter : x) p(iter)
#define yes(x) x ? cout << "Yes" << endl : cout << "No" << endl;
using namespace std;
typedef long long lint;
typedef long long LL;
typedef long double LD;
typedef vector<int> VI;
typedef vector<long long> VLL;
typedef vector<vector<int> > VVI;
typedef vector<vector<long long> > VVLL;
typedef vector<vector<bool> > VVB;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<PII> VPII;
typedef vector<PLL> VPLL;
using UnweightedGraph = vector<vector<int> >;
using WeightedGraph = vector<vector<pair<int, int> > >;
const long long INF = 2000000000ll;
const long double PI = acosl(-1);
const long double EPS = 1e-7;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }



long long power(long long a, long long b) {return b?power(a*a,b/2)*(b%2?a:1):1;}
long long perm(long long N, long long K){
    long long x=1;
    for(long long i=1;i<=K;i++)
    {
        x=x*(N-i+1);
    }
    return x;
}
long long factorial(long long N) {return perm(N,N);}
long long comb(long long N,long long K) {return factorial(N)/factorial(K)/factorial(N-K);}
long long hcomb(long long N, long long K) {return comb(N+K-1, N-1);}

int dtoi(double d) {return round(d);}

bool isprime(int n) {
    if (n < 2) return false;
    for (int i = 2; i*i <= n; ++i) {
        if (n % i ==  0) return false;
    }
    return true;
}

long long mystoll(string s){
    long long ans = 0;
    for (int i = (int) s.length()-1; i >= 0; i--){
        ans += (s[i]-'0') * power(10,s.length()-1-i);
    }
    return ans;
}

template<class T>
long long SUM(T a) {
    long long sum = 0;
    for(auto itr : a) sum += itr;
    return sum;
}

void solve(int64_t N, int M, const vector<int64_t> & A) {
    N-SUM(A)<0 ? p(-1) : p(N-SUM(A));
}

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cout << fixed << setprecision(10);
    int N;
    int M;
    cin >> N >> M;
    vector<int64_t> A(M);
    REP (i, M) {
        cin >> A[i];
    }
    solve(N, M, A);
    return 0;
}
/*
cd ~/Desktop/AtCoder/contest/abc163/b && sh test.sh
cd ~/Desktop/AtCoder/contest/abc163/b && sh etest.sh
cd ~/Desktop/AtCoder/contest/abc163/b && sh submit.sh
PLEASE TYPE : abcb
*/


Measured memory_bytes: 3736