#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <cmath>
#include <queue>
#include <numeric>
#include <bitset>

using namespace std;
#define repp(i,a,b) for(int i = (int)(a) ; i < (int)(b) ; ++i)
#define reppe(i,a,b) for(int i = (int)(a) ; i <= (int)(b) ; ++i)
#define repm(i,a,b) for(int i = (int)(a) ; i > (int)(b) ; --i)
#define repme(i,a,b) for(int i = (int)(a) ; i >= (int)(b) ; --i)

typedef long long ll;

long long GCD(long long a, long long b){if(b==0)return a;return GCD(b,a%b);}

int main() {
    long long N, M; cin >> N >> M;
    vector<long long> A(M); repp(i,0,M) cin >> A[i];
    long long sum_ = 0;
    repp(i,0,M) sum_ += A[i];
    cout << max(-1LL, N - sum_) << endl;
}
