#include <bits/stdc++.h>
using namespace std;
using int64 = long long;
#define int int64
#define rep(i, n) for(int i=0; i<n; i++)
#define FOR(i, a, b) for(int i=a; i<b; i++)
#define SORT(x) sort(x.begin(), x.end())
#define GSORT(x) sort(x.begin(), x.end(), greater<int>())
#define mk make_pair
#define fi first
#define se second
#define pb push_back
#define ALL(x) x.begin(), x.end()
#define V(T) vector<T>
typedef pair<int, int> P;
typedef pair<P, P> PP;
typedef vector<int> vi;
typedef vector<vi> vvi;
int max(int a, int b) {if(b>a) return b; else return a;}
int min(int a, int b) {if(b<a) return b; else return a;}


signed main() {
    int N, M;
    cin >> N >> M;
    int sum = 0;
    rep(i, M) {
        int a;
        cin >> a;
        sum += a;
    }

    cout << (N>=sum ? N-sum : -1) << endl;


    return 0;
}