#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
using namespace std;
using ll = long long;
const int INF = 1001001001;
const int MOD = 1000000007;
typedef long long ll;

template<typename T> void print(const T & v);


int main() {
    int N, M;
    cin >> N >> M;
    vector<int> A(M);
    rep(i, M) cin >> A[i];

    rep(i, M){
        N -= A[i];
    }
    if(N >= 0){
        cout << N << endl;
    }else{
        cout << -1 << endl;
    }
    return 0;
}






// Use For Debug
template <typename T>
void print(T const & v){
    for(int i = 0; i < v.size(); i++){
        if(i) cout << " ";
        cout << v[i];
    }
    cout << endl;
}
