#include <iostream>
#include <math.h>
#include <algorithm>
#include <string>
#include <string.h>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <tuple>
#include <stdlib.h>
#define rep(i,n) for (int i = 1; i <= (n); ++i)
using namespace std;
#define PI 3.14159265358979
typedef long long int ll;

const double pi=3.1415;
ll absc(ll a,ll b){return (a-b>0?a-b:b-a);
}
ll mini(ll a,ll b){return (a>b?b:a);}
ll gcd(ll a,ll b){
	if(a<b)swap(a,b);
	return (a%b?gcd(a%b,b):b);
}
int main()
{
    ll a,b,c;
    string s,t;
    cin>>a>>b;
    rep(i,b){
    	cin>>c;
    	a-=c;
    }
    cout<<(a>=0?a:-1);
}
