/* ****TCIITB**** */

#include<bits/stdc++.h>
#define io          ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0); 
#define ll          long long
#define pb          push_back
#define    endl        '\n'
#define pii         pair<ll int,ll int>
#define vi          vector<ll int>
#define all(a)      (a).begin(),(a).end()
#define F           first
#define S           second
#define sz(x)       (ll int)x.size()
#define hell        1000000007
#define rep(i,a,b)    for(ll int i=a;i<b;i++)
#define lbnd        lower_bound
#define ubnd        upper_bound
#define bs          binary_search
#define mp          make_pair
#define time        cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
using namespace std;

#define N  100005

int main()
{
    
    io

    ll n,m;
    cin>>n>>m;
    ll ts=0;
    rep(i,0,m)
    {
        ll x;
        cin>>x;
        ts+=x;
    }
    if(ts>n)
        cout<<"-1";
    else
        cout<<(n-ts);
    
}