#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace __gnu_pbds; 
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update> 
#define ll long long
#define ld long double
#define MAX 1000000000
int main(){
	ios_base::sync_with_stdio(false);
    cin.tie(NULL);
	int t=1;
	//cin>>t;
	for(int cc=1;cc<=t;cc++){
        //cout<<"Case #"<<cc<<": ";
        ll n,m,ans=0;
        cin>>n>>m;
        ll a[m];
        for(int i=0;i<m;i++)
        	cin>>a[i],ans+=a[i];
        cout<<max(-1LL,n-ans)<<endl;
	}
}
