/***************************************** START OF TEMPLATE *********************************************/

#include<bits/stdc++.h>

using namespace std;

#define  SPEED          ios:: sync_with_stdio(0);cin.tie(0);cout.tie(0);cout<<fixed;cout<<setprecision(10);
#define  randomINIT     mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
#define  foo(i,a,n)     for(ll i = (a); i <= n; i++)
#define  frr(i,a,n)     for(ll i = (a); i >= n; i--)
#define  fo(i,n)        for(ll i = 0; i < n; i++)
#define  all(x)         (x).begin(),(x).end()
#define  mset(x,val)    memset(x,val,sizeof(x))
#define  newl           cout<<"\n"
#define  f              first
#define  s              second
#define  pb             push_back
#define  INPa(a,n)      foo(i,1,n) cin>>a[i];
#define  deb2d(a,n,m)   foo(i,1,n){foo(j,1,m){cout<<a[i][j]<<" ";}newl;}    
#define  deb1d(a,n)     foo(i,1,n)cout<<a[i]<<" ";newl;
#define  debv(a)        for(auto it: a)cout<<it<<" ";newl;
#define  deb1(a)        cout<<a<<" ";newl;
#define  deb2(a,b)      cout<<a<<" "<<b<<" ";newl;
#define  deb3(a,b,c)    cout<<a<<" "<<b<<" "<<c<<" ";newl;
#define  deb4(a,b,c,d)  cout<<a<<" "<<b<<" "<<c<<" "<<d;newl;
#define  ll             long long
#define  ld             long double
#define  pll            pair<ll,ll>
#define  vll            vector<ll>
#define  vpll           vector<pll>

const ll   MOD   =  1e+9+7;
const ll   INF   =  LLONG_MAX;
const int  INFi  =  INT_MAX;
const ll   N     =  2e+5+8;

vll adj[N]; ll vis[N];
ll dx4[]={0,1,0,-1}, dy4[]={1,0,-1,0};

ll test_cases = 1; randomINIT;

/*********************************************************************************************************/
/***************************************** VARIABLE DECLARATIONS *****************************************/
ll a[N];


/***************************************** FUNCTION IMPLEMENTATIONS **************************************/



/***************************************** START OF MAIN FUNCTION ****************************************/
void MAIN(ll tc){
    ll n, m, sum =0;
    cin>>n>>m;
    INPa(a,n);
    foo(i,1,m){
        sum += a[i];
    }
    if(sum>n){
        cout<<-1;
    }else{
        cout<<n-sum;
    }
}

int main(){
    SPEED;

    //cin>>test_cases;
    foo(i,1,test_cases){
        MAIN(i);
    }
}