#include <iostream>
using namespace std;
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);

#define ull unsigned long long int
#define ll long long int
#define ld double
#define ui unsigned int
#define pb push_back
#define mp make_pair
#define ff first
#define ss second 

#define fr(i,a,b) for (int i = (a), _b = (b); i <= _b; i++)
#define frr(i,a,b) for (int i = (a), _b = (b); i >= _b; i--)
#define rep(i,n) for (int i = 0, _n = (n); i < _n; i++)
#define repr(i,n) for (long long i = (n) - 1; i >= 0; i--)


ll gcd(ll a, ll b) 
{ 
    if (a == 0) return b; 
    return gcd(b % a, a); 
} 

int main(){

    ll t; t=1;
    while(t--){
        ll n,m; cin>>n>>m;
        ll a[m];
        ll sum=0;
        rep(i,m){
            cin>>a[i];
            sum+=a[i];
        }
        if(n-sum<0) cout<<-1;
        else cout<<n-sum;
    }
    return 0;
}