/*

 ALLAH IS ALMIGHTY              ******BISMILLAHIR RAHMANIR RAHIM.******             ALLAH IS ALMIGHTY
                                    ```````````````````````````````
                                        AUTHOR: ANISUR RAHAMAN
                                            BSMRSTU(SHIICT)
                                                ID:017

                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                           ****!!!!!!!!****
                                            **************
                                             ************
                                              **********
                                               ********
                                                ******
                                                 ****
                                                  **

 _____________________________________________________________________________________________________
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 `````````````````````````````````````````````````````````````````````````````````````````````````````
 ******                                                                                         ******
 *****                                                                                           *****
 ****                                                                                             ****
 ***                                                                                               ***
 **                                                                                                 **
 *                                                                                                   *
         $$$$$$$$$$$$$$         $$$$$$           $$$$$$$$    $$$$$$$$$$        $$$$$$$$$$$$$$$
       $$$$$$$$$$$$$$$$$$       $$$$$$$           $$$$$$    $$$$$$$$$$$$      $$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$ $$$           $$$$    $$$$$$$$$$$$$$    $$$$$$$$$$$$$$$$$$$
      $$$$            $$$$      $$$$  $$$          $$$$         $$$$         $$$$
      $$$$            $$$$      $$$$   $$$         $$$$         $$$$         $$$$
      $$$$            $$$$      $$$$    $$$        $$$$         $$$$         $$$$
      $$$$     $$     $$$$      $$$$     $$$       $$$$         $$$$         $$$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$      $$$      $$$$         $$$$          $$$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$       $$$     $$$$         $$$$           $$$$$$$$$$$$$$$$$$
      $$$$            $$$$      $$$$        $$$    $$$$         $$$$                         $$$$
      $$$$            $$$$      $$$$         $$$   $$$$         $$$$                         $$$$
      $$$$            $$$$      $$$$          $$$  $$$$         $$$$                         $$$$
      $$$$            $$$$      $$$$           $$$ $$$$    $$$$$$$$$$$$$$     $$$$$$$$$$$$$$$$$$$
     $$$$$$          $$$$$$    $$$$$$           $$$$$$$     $$$$$$$$$$$$       $$$$$$$$$$$$$$$$$
    $$$$$$$$        $$$$$$$$  $$$$$$$$           $$$$$$      $$$$$$$$$$         $$$$$$$$$$$$$$$

                        LETS START OUR JOURNEY...................

*/

#include<bits/stdc++.h>
#define pb push_back
#define pf push_front
#define in insert
#define ff first
#define ss second
#define off return 0
#define mp make_pair
#define MOD 1000000007
#define ll long long int
#define ld long double
#define snll(a) scanf("%lld", &a)
#define snll2(a,b) scanf("%lld %lld", &a,&b)
#define snll3(a,b,c) scanf("%lld %lld %lld", &a,&b,&c)
#define snl(b) scanf("%d", &b)
#define snl2(a,b) scanf("%d %d", &a,&b)
#define snl3(a,b,c) scanf("%d %d %d", &a,&b,&c)
#define ptl(a) printf("%d\n", a)
#define ptll(a) printf("%lld\n", a)
#define ptl2(a,b) printf("%d %d\n", a,b)
#define ptll2(a,b) printf("%lld %lld\n", a,b)
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) (a*b)/gcd(a,b)
#define pi acos(-1)
#define C continue
#define al(v) v.begin(),v.end()
#define sorts(v) sort(v.begin(),v.end())
#define rev(v) reverse(v.begin(),v.end())
#define loop(i,a,n) for(int i=a;i<n;i++)
#define T int n; scanf("%d", &n); while(n--)
using namespace std;
int main()
{
    int n,m,s=0;
    cin>>n>>m;
    int arr[m];
    for(int i=0; i<m; i++)
    {
        cin>>arr[i];
        s+=arr[i];
    }
    if(s<=n) cout << n-s << endl;
    else cout << "-1" << endl;
    return 0;
}
/*                                     *******THE END********                                     */



Measured memory_bytes: 3672