//BUGSBUNNY
//IIT ROORKEE
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/detail/standard_policies.hpp>
using namespace std; 
using namespace __gnu_pbds;
#define int long long
#define ll long long
#define double long double
#define ld long double
#define endl '\n'
#define vi vector<int>
#define vvi vector<vi>
#define vs vector<string>
#define vl vector<long long>
#define vpii vector<pii>
#define vpipii vector<pipii>
#define vb vector<bool>
#define pb push_back
#define pob pop_back
#define mp make_pair
#define pii pair<int,int>
#define pipii pair<int,pii>
#define piipi pair<pii,int>
#define pld pair<long double,long double>
#define mod 1000000007 
// #define mod 998244353
#define rep(i,n) for (ll i = 0; i < n; i++)
#define repp(i,a,b) for(ll i = a ; i<b ; i++)
#define reppr(i,a,b) for(ll i = a-1 ; i>=b ; i--)
#define repr(i,n) for (ll i = n - 1; i >= 0; i--)
#define ff first
#define ss second
#define pc putchar_unlocked
#define gc getchar_unlocked
#define inf 9223372036854775807
#define infn -9223372036854775807
#define pi 3.14159265358979323846
#define eps 0.0000000001
#define sp << " " <<
#define setprec(x) cout << fixed << setprecision(x);
#define REVERSE(a) reverse(all(a));
#define SORT(a) sort(all(a));
#define all(n) n.begin(),n.end()

//GCD and LCM
    int gcd (int a, int b) { return b ? gcd (b, a % b) : a; }
    int lcm (int a, int b) { return a / gcd(a, b) * b; }

//Modular Exponentiation
    int powmod(int x,int y) 
    { 
        if (y == 0) return 1; 
        int p = powmod(x, y/2) % mod; 
        p = (p * p) % mod;   
        return (y%2 == 0)? p : (x * p) % mod; 
    }

//Modular Inverse
    int inverse(int a) 
    { 
       return powmod(a,mod-2); 
    }    


//Binomial Coefficients
    int fact[200010], factinv[200010] , first = 0;
    void Util_nCr()
    {
        fact[0] = 1 , factinv[0] = 1;
        repp(i,1,200010)
        {
            fact[i] = (fact[i-1] * i)%mod;
            factinv[i] = (factinv[i-1] * inverse(i))%mod;
        }
    }
    int nCr(int n , int r)
    {
        if(!first) first = 1 , Util_nCr();             
        return ((fact[n] * inverse(fact[n-r]) % mod * inverse(fact[r]))%mod) ;
    }

void solve();

signed main()
{
    //-------------------------------------------------------------------------------   
        ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
        clock_t clk = clock();
        
    //-------------------------------------------------------------------------------   

    //-------------------------------------------------------------------------------
        int t = 1 , codejam = 0;
        // cin >> t;    
        rep(i,t)
        {
            if(codejam)
                cout << "Case #" << i+1 << ": ";
            solve();
        }
    //-------------------------------------------------------------------------------
        clk = clock() - clk;
        cerr << fixed << setprecision(12) << "Time: " << ((double)clk)/CLOCKS_PER_SEC;
    //-------------------------------------------------------------------------------
}

void solve()
{
    int n , m , sum = 0 , temp;
    cin >> n >> m; 
    rep(i,m)
        cin >> temp , sum += temp;
    if(sum>n)
        cout << -1;
    else
        cout << n-sum;
}

Measured memory_bytes: 3812