/// Author : HASEBUL HASSAN TANIM ///
#include <bits/stdc++.h>
using namespace std;

#define fast ios_base::sync_with_stdio(0);cin.tie(0);
#define pb push_back
#define pp pop_back
#define scan(t) scanf("%d",&t)
#define sscan(t) scanf("%lld",&t)
#define MEM(a, b) memset(a, (b), sizeof(a))
#define LL long long
#define ull unsigned long long
#define rock(cont) cont.begin(), cont.end()
#define roll(a,n) arr,arr+n
#define rep(i,n) for(int i=0 ; i<n ; i++)
#define repp(i, n) for(int i=1 ; i<=n ; i++)
#define print(t) printf("%d",t)
#define pprint(t) printf("%lld",t)
#define co(x) cout<<x
#define nl printf("\n")
#define el cout<<endl
#define INF (int)1e9

const int RT = 10000000;

vector<int>prime;
vector<LL>factor;
bitset<10000003> status;



int gcd(LL a,LL b){ return b==0 ? a: gcd(b,a%b);}
int lcm(LL a,LL b) { return a* (b/gcd(a,b));}

vector<int> Divisors(int n)
{
    vector<int> v;
    int limit=sqrt(n);
    for (int i=1; i<=limit; i++)
    {
        if (n%i==0)
        {
            if (n/i == i)
                v.push_back(i);
            else
            {
                v.push_back(i);
                v.push_back(n/i);
            }
        }
    }
    return v;
}

void sieve()
{
    int rt = sqrt(RT);
    //for(int i = 4; i <= rt; i+=2) status[i]=true;
    for(int i = 3; i <= rt; i+=2)
        if(!status[i])
            for(int j = i*i; j < RT; j+=(i<<1))
                status[j] = true;

    prime.push_back(2);

    for(int i = 3; i <= RT; i+=2)
        if(!status[i])
            prime.push_back(i);
}

LL primefac(LL n)
{
    int pf_idx=0,pf=prime[pf_idx],SIZE=prime.size();
    LL ANS=1,step=0;
    while(pf*pf<=n&&pf<SIZE)
    {
        if(n%pf==0)
        {
            while(n%pf==0) {n/=pf;}
            ANS=pf;
            step++;
        }
        pf=prime[++pf_idx];
        //cout<<pf<<" "<<n<<" "<<ANS<<endl;
    }
    if(n!=1) { ANS=max(ANS,n); step++; }
    if(step>1)
    {
      return ANS;
    }
    else
    {
        return -1;
    }
}

int main()
{
    int n,m,x,sum=0;
    scan(n);
    scan(m);
    repp(i,m)
    {
        scan(x);
        sum+=x;
    }
    if(sum>n)
    {
        print(-1);
        el;
    }
    else
    {
        print(n-sum);
        el;
    }
}
