#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define l long
#define pb push_back
#define mp make_pair
#define mt make_tuple
int main() {
	// your code goes here
	//ifstream cin("input.txt");
	//ofstream cout("output.txt");
	ios_base::sync_with_stdio(false); 
    cin.tie(NULL); 
	ll n,m; cin>>n>>m;
	for(ll i=0;i<m;i++)
	{
	    ll p; cin>>p;
	    n -= p;
	}
	if(n>=0) cout<<n;
	else cout<<-1;
	
	return 0;
}
