#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <deque>
using namespace std;
using P = pair<int,int>;
typedef long long int ll;
#define rep(i, n) for (int(i) = 0; (i) < (n); (i)++)
#define PI 3.14159265359
ll gcd(ll a, ll b) {//最大公約数
    if(a<b){
        ll tmp = a;
        a = b;
        b = tmp; 
    }
    if (b==0) return a;
    else return gcd(b, a%b);
}
ll lcm(ll a, ll b) {//最小公倍数
    return a * b / gcd(a, b);
}

int main() {
    int n,m,work=0;
    cin >> n>> m;
    rep(i,m){
        int a;
        cin >> a;
        work+=a;
    }
    int ans = n-work;
    if(ans<0){
        cout << -1 << endl;
    }else{
        cout << ans << endl;
    }

}


