#include<bits/stdc++.h>
   using namespace std;
   //
   typedef long double ld;
   typedef long long int ll;
   //
   typedef pair<long long int,long long int> pii;
   typedef pair<ll,ll> pll;
   //
   typedef vector<long long int> vi;
   typedef vector<ll> vl;
   typedef vector<char> vc;
   typedef vector<pii> vii;
   typedef vector<pll> vll;
   typedef vector<vi> vvi;
   typedef vector<vl> vvl;
   #define prq priority_queue<ll>;
   #define vbs(xx,x) binary_search(all(xx),x)
   #define vlb(xx,x) lower_bound(all(xx),x)
   #define vub(xx,x) upper_bound(all(xx),x)
   //
   //ll MOD=998244353;
   ll M=1e9+7;
   ll MOD=1e9+7;
   #define PI 3.1415926535897932384626433832795
   ll llMAX=1000000000000000008;
   ll llMIN=-1000000000000000008;
   //
   #define forx(i,j,n) for(long long int i=j;i<n;i++)
   #define fory(i,j,n) for(long long int i=j;i>=n;i--)
   #define all(x) x.begin(),x.end()
   #define pb push_back
   #define mp make_pair
   #define fs first
   #define sn second
   #define MEM(xx,yy) memset(xx,yy,sizeof(xx))
   //
   #define in(x) cin>>x
   #define op(x) cout<<x
   #define opnl(x) cout<<x<<"\n"
   #define ops(x) cout<<x<<" "
   #define nl cout<<"\n"
   #define sp cout<<" "
   #define flush fflush(stdout)
   clock_t tim;
   void showtime(){ tim=clock()-tim; cout<<" ===> "<<(ld)tim/CLOCKS_PER_SEC<<" seconds\n"; }
   //
   ll mul(ll mul_1,ll mul_2){ ll mul_t=((mul_1%MOD)*(mul_2%MOD)+MOD)%MOD; return mul_t; }
   ll mul3(ll mul_1,ll mul_2,ll mul_3){ ll mul_t=mul(mul(mul_1,mul_2),mul_3)%MOD; return mul_t; }
   ll add(ll add_1,ll add_2){ ll add_t=((add_1%MOD)+(add_2%MOD)+MOD+MOD)%MOD; return add_t; }
   ll sub(ll sub_1,ll sub_2){ ll sub_r=1e15; ll sub_t=(((sub_1+sub_r)%MOD)-((sub_2+sub_r)%MOD)+sub_r)%MOD; return sub_t; }
   ll pwm(ll x,ll y,ll p=M){ ll res=1; x=x%p; while(y>0){ if(y&1) res=(res*x)%p; y=y/2; x=(x*x)%p; } return res; } 
   ll pw(ll x,ll y){ ll res=1; while(y>0){ if(y&1) res=(res*x); y=y/2; x=(x*x); } return res; } 
   ll modinv(ll n){ return pwm(n, M-2, M); }
   ll gcd(ll num1,ll num2){ return (num2?gcd(num2,num1%num2):num1); }
   ll lcm(ll num1,ll num2){ return 1LL*((num1*num2)/__gcd(num1,num2)); }
   ll myceil(ll x,ll y){ return (x+y-1)/y;}
   long long int my_log(ll n, long long int b){ll i = 1;long long int ans = 0;while (1){if (i > n){ans--;break;}if (i == n)break;i *= b;ans++;}return ans;}
   //
   ll countbit(ll num){ return 1LL*(__builtin_popcountll(num)); }
   ll setbit(ll num, ll kk){ return 1LL*(num|(1LL<<kk)); }
   ll unsetbit(ll num, ll kk){ return 1LL*(num&~(1LL<<kk)); }
   ll invertbit(ll num, ll kk){ return 1LL*(num^(1LL<<kk)); }
   bool kthbit(ll num, ll kk){ return num&(1LL<<kk); }
   ll zerotrail(ll num){ return 1LL*(__builtin_ctzll(num)); }
   ll zerobegin(ll num){ return 1LL*(__builtin_clzll(num)); }
   //
   ll prime[1000005]; 
   void sieve(){ //spf
      forx(i,0,1000005) prime[i]=i; 
      for(ll p=2; 1LL*p*p<1000005; p++){ if(prime[p]==p) { for(ll i=1LL*p*p; i<1000005; i+=p) if(prime[i]==i) prime[i]=p; } } 
      return; 
   } 
   bool isPrime(ll pri){
      if(pri==1||pri==0) return 0;
      if(prime[pri]==pri) return 1;
      return 0;
   }
   vl alldiv(ll num){
      vl prDiv; for(ll i=1; 1LL*i*i<=num; i++){ if(num%i==0){ prDiv.pb(i); if(i!=(num/i)) prDiv.pb(num/i); } }
      sort(all(prDiv)); return prDiv;
   }
   vl prdiv(ll num){
      vl prDiv; while(num!=1){ prDiv.pb(prime[num]); ll prtemp=prime[num]; while(num%prtemp==0) num/=prtemp; }
      return prDiv;
   }
   //
   /*
   bool vis[100005];
   int dp[100005];
   vector<vector<int>>ad(100005);
   void dfs(int root){
      vis[root]=true;
      for(auto j:ad[root]){
         if(!vis[j]){
            dfs(j);
         }
         dp[root]=max(dp[root],1+dp[j]);
      }
   }
   */

   //
   int main(){
   ios_base::sync_with_stdio(false);
   cin.tie(NULL);

   long long int x=1;
   //comment for single test case
   //in(x);
   forx(u,0,x){
      //cout<<"Case #"<<u+1<<":"<<" ";
      long long int n,k,m,f,q,p,b,c;
      cin>>n>>k;
      b=0;
      for(int i=0;i<k;i++){
         cin>>p;
         b+=p;
      }
      if(b>n) cout<<-1<<"\n";
      else cout<<n-b<<"\n";
      
         
   }

      //opnl(r);
   //showtime();
   return 0;
   }


Measured memory_bytes: 3660