Problem:
Score : 200 points

Problem Statement

Takahashi has N days of summer vacation.

His teacher gave him M summer assignments. It will take A_i days for him to do the i-th assignment.

He cannot do multiple assignments on the same day, or hang out on a day he does an assignment.

What is the maximum number of days Takahashi can hang out during the vacation if he finishes all the assignments during this vacation?

If Takahashi cannot finish all the assignments during the vacation, print -1 instead.

Constraints

1 \leq N \leq 10^6

1 \leq M \leq 10^4

1 \leq A_i \leq 10^4

Input

Input is given from Standard Input in the following format:

N M
A_1 ... A_M

Output

Print the maximum number of days Takahashi can hang out during the vacation, or -1.

Sample Input 1

41 2
5 6

Sample Output 1

30

For example, he can do the first assignment on the first 5 days, hang out on the next 30 days, and do the second assignment on the last 6 days of the vacation. In this way, he can safely spend 30 days hanging out.

Sample Input 2

10 2
5 6

Sample Output 2

-1

He cannot finish his assignments.

Sample Input 3

11 2
5 6

Sample Output 3

0

He can finish his assignments, but he will have no time to hang out.

Sample Input 4

314 15
9 26 5 35 8 9 79 3 23 8 46 2 6 43 3

Sample Output 4

9

Solution in Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.*;
import java.io.*;

public class Main{
  static int repow(int b,int p){
    long a = b;
    long res=1;
    while(p>0){
      if(p%2==1){
        res*=a;
      }
      a*=a;
      p/=2;
    }
    return (int)res;
  }
  static int repow(int b,int p,int modder){
    long a = b%modder;
    long res=1;
    while(p>0){
      if(p%2==1){
        res=(res*a)%modder;
      }
      a=(a*a)%modder;
      p/=2;
    }
    return (int)res;
  }
  static long repow(long b,long p){
    long a = b;
    long res=1;
    while(p>0){
      if(p%2==1){
        res*=a;
      }
      a*=a;
      p/=2;
    }
    return res;
  }
  static long repow(long b,long p,long modder){
    long a = b%modder;
    long res=1;
    while(p>0){
      if(p%2==1){
        res=(res*a)%modder;
      }
      a=(a*a)%modder;
      p/=2;
    }
    return res;
  }
  static long gcd(long c,long d){
    while(true){
      long f=c%d;
      if(f==0){
        return d;
      }
      c=d;
      d=f;
    }
  }
  static long lcm(long c,long d){
    return c/gcd(c,d)*d;
  }
  static ArrayList<Integer> divList(int n){
    ArrayList<Integer> div=new ArrayList<Integer>();
    for(int i=1;i*i<=n;i++){
      if(n%i==0){
        div.add(i);
        if(i*i!=n){
          div.add((int)(n/i));
        }
      }
    }
    return div;
  }
  static ArrayList<Long> divList(long n){
    ArrayList<Long> div=new ArrayList<Long>();
    for(long i=1;i*i<=n;i++){
      if(n%i==0){
        div.add(i);
        if(i*i!=n){
          div.add(n/i);
        }
      }
    }
    return div;
  }
  static HashSet<Integer> divSet(int n){
    HashSet<Integer> div=new HashSet<Integer>();
    for(int i=1;i*i<=n;i++){
      if(n%i==0){
        div.add(i);
        div.add((int)(n/i));
      }
    }
    return div;
  }
  static Map<Long,Integer> pFacMap(long x){
    Map<Long,Integer> mp=new HashMap<>();
    long t=x;
    for(long i=2;i*i<=x&&i<=t;i++){
      if(t%i==0){
        int num=0;
        while(t%i==0){
          t/=i;
          num++;
        }
        mp.put(i,num);
      }
    }
    if(t!=1){
      mp.put(t,1);
    }
    return mp;
  }
  static boolean iP(long n){
    if(n==2){
      return true;
    }
    if((n&1)==0||n==1){
      return false;
    }
    //    if(n>3037007383L){
    if(n>Integer.MAX_VALUE){
      return tameshiwari(n);
    }
    long d=n-1;
    int s=0;
    while((d&1)==0){
      d/=2;
      s++;
    }
    int[] aa = {2,3,5,7,11,13,17};
    for(int i=0;i<7&&aa[i]<n;i++){
      int a = aa[i];
      long t = d;
      long y=repow(a,t,n);
      while(t!=n-1&&y!=1&&y!=n-1){
        y = (y*y)%n;
        t=t<<1;
      }
      if(y!=n-1&&(t&1)==0){
        return false;
      }
    }
    return true;
  }
  static boolean tameshiwari(long n){
    for(long i=2;i*i<=n;i++){
      if(n%i==0){
        return false;
      }
    }
    return true;
  }
  static class NCK{
    int max;
    int mod;
    long[] fac;
    long[] finv;
    long[] inv;
    NCK(){
      this(510000,1000000007);
    }
    NCK(int max,int mod){
      this.max=max;
      this.mod=mod;
      pre(max,mod);
    }
    void pre(int nmax,int nmod){
      fac = new long[nmax];
      finv = new long[nmax];
      inv = new long[nmax];
      fac[0]=fac[1]=1;
      finv[0]=finv[1]=1;
      inv[1]=1;
      for(int i=2;i<nmax;i++){
        fac[i]=fac[i-1]*i%nmod;
        inv[i]=nmod-inv[nmod%i]*(nmod/i)%nmod;
        finv[i]=finv[i-1]*inv[i]%nmod;
      }
    }
    long nCk(int n,int k){
      if(n<k){return 0;}
      if(n<0||k<0){return 0;}
      return fac[n]*(finv[k]*finv[n-k]%mod)%mod;
    }
  }
  static int kmp(String t,String p){
    int[] f=new int[p.length()+1];
    int i=0;
    int j=1;
    f[1]=0;
    while(j<p.length()){
      if(i==0||p.charAt(i-1)==p.charAt(j-1)){
        i++;
        j++;
        f[j]=i;
      }else{
        i=f[i];
      }
    }
    i=1;
    j=1;
    while(i<=p.length()&&j<=t.length()){
      if(i==0||p.charAt(i-1)==t.charAt(j-1)){
        i++;
        j++;
      }else{
        i=f[i];
      }
    }
    return i==(p.length()+1)?j-i:-1;
  }
  static String StSort(String s){
    StringBuilder sb = new StringBuilder(s);
    int lg = sb.length();
    int l;
    int r;
    int gaze;
    for(int i=1;i<lg;i++){
      l=0;
      r=i-1;
      while(l<=r){
        gaze=(l+r)/2;
        if(sb.charAt(gaze)<=sb.charAt(i)){
          l=gaze+1;
        }else if(sb.charAt(gaze)>sb.charAt(i)){
          r=gaze-1;
        }
      }
      sb.insert(l,sb.charAt(i));
      sb.deleteCharAt(i+1);
    }
    return sb.toString();
  }
  static class Xy{
    int x;
    int y;
    Xy(int x,int y){
      this.x=x;
      this.y=y;
    }
    public int manht(Xy o){
      return Math.abs(x-o.x)+Math.abs(y-o.y);
    }
    public String toString(){
      return "["+x+","+y+"]";
    }
    public double henkaku(){
      return Math.atan2(y,x);
    }
    public static int hencom(Xy s1,Xy s2){
      return (int)Math.signum(s1.henkaku()-s2.henkaku());
    }
    public boolean equals(Object o){
      return x==((Xy)o).x&&y==((Xy)o).y;
    }
  }
  static class Zip1{
    Map<Long,Integer> zip=new HashMap<>();
    long[] unzip;
    Zip1(long[] a){
      Arrays.sort(a);
      long mae=0;int zure=0;
      for(int i=0;i<a.length;i++){
        if(i==0||mae!=a[i]){
          zip.put(a[i],i-zure);
          mae=a[i];
        }else{
          zure++;
        }
      }
      unzip=new long[size()];
      for(Map.Entry<Long,Integer> me:zip.entrySet()){
        unzip[me.getValue()]=me.getKey();
      }
    }
    int zip(long t){
      return zip.get(t);
    }
    long unzip(int i){
      return unzip[i];
    }
    int size(){
      return zip.size();
    }
  }
  static class UnFd{
    int[] parent;
    int[] size;
    int forest;
    UnFd(int n){
      parent=new int[n];
      size=new int[n];
      forest=n;
      for(int i=0;i<n;i++){
        parent[i]=i;
        size[i]=1;
      }
    }
    private boolean isRoot(int i){
      return parent[i]==i;
    }
    private int rootOf(int i){
      if(isRoot(i)){
        return i;
      }
      return parent[i]=rootOf(parent[i]);
    }
    public boolean same(int i,int j){
      return rootOf(i)==rootOf(j);
    }
    public void merge(int i,int j){
      i=rootOf(i);
      j=rootOf(j);
      if(i==j){
        return;
      }
      size[Integer.min(i,j)]+=size[Integer.max(i,j)];
      parent[Integer.max(i,j)]=Integer.min(i,j);
      forest--;
    }
    public int sizeOf(int i){
      return size[rootOf(i)];
    }
    public int[] parents(){
      return parent;
    }
    public int forest(){
      return forest;
    }
    public String toString(){
      return Arrays.toString(parent);
    }
  }
  static class SegTree<T>{
    int n;
    int m;
    java.util.function.BinaryOperator<T> op;
    int[] l;
    int[] r;
    T[] a;
    T ident;
    SegTree(int n,java.util.function.BinaryOperator<T> op,T ident){
      this.n=n;
      this.op=op;
      this.ident=ident;
      int ii=n-1;
      int p=0;
      while(ii>0){
        ii/=2;
        p++;
      }
      m=repow(2,p+1)-1;
      @SuppressWarnings("unchecked")
      T[] b=(T[])(new Object[m]);
      a=b;
      Arrays.fill(a,ident);
      l=new int[m];
      r=new int[m];
      for(int i=0;i<=m/2;i++){
        l[m/2+i]=i;
        r[m/2+i]=i+1;
      }
      for(int i=m/2-1;i>=0;i--){
        l[i]=l[lch(i)];
        r[i]=r[rch(i)];
      }
    }
    SegTree(int n,java.util.function.BinaryOperator<T> op,T ident,T[] ary){
      this.n=n;
      this.op=op;
      this.ident=ident;
      int ii=n-1;
      int p=0;
      while(ii>0){
        ii/=2;
        p++;
      }
      m=repow(2,p+1)-1;
      @SuppressWarnings("unchecked")
      T[] b=(T[])(new Object[m]);
      a=b;
      for(int i=0;i<n;i++){
        a[m/2+i]=ary[i];
      }
      for(int i=m/2+n;i<m;i++){
        a[i]=ident;
      }
      for(int i=m/2-1;i>=0;i--){
        a[i]=op.apply(a[lch(i)],a[rch(i)]);
      }
      l=new int[m];
      r=new int[m];
      for(int i=0;i<=m/2;i++){
        l[m/2+i]=i;
        r[m/2+i]=i+1;
      }
      for(int i=m/2-1;i>=0;i--){
        l[i]=l[lch(i)];
        r[i]=r[rch(i)];
      }
    }
    public T getAll(){
      return a[0];
    }
    public T get(int from,int to){
      if(from<0||n<to||from>=to){
        throw new IllegalArgumentException(String.valueOf(from)+","+String.valueOf(to));
      }
      return get(from,to,0);
    }
    private T get(int from,int to,int node){
      if(from==l[node]&&to==r[node]){
        return a[node];
      }else{
        if(to<=l[node]+(r[node]-l[node])/2){
          return get(from,to,lch(node));
        }else if(l[node]+(r[node]-l[node])/2<=from){
          return get(from,to,rch(node));
        }else{
          return op.apply(get(from,l[node]+(r[node]-l[node])/2,lch(node)),get(l[node]+(r[node]-l[node])/2,to,rch(node)));
        }
      }
    }
    public void set(T ob,int i){
      if(i<0||n<=i){
        throw new IndexOutOfBoundsException(String.valueOf(i)+"isOutFromLength "+String.valueOf(n));
      }
      int j=m/2+i;
      a[j]=ob;
      while(true){
        if(j==0){
          break;
        }
        j=prt(j);
        a[j]=op.apply(a[lch(j)],a[rch(j)]);
      }
    }
    static private int prt(int node){
      return (node-1)/2;
    }
    static private int lch(int node){
      return 2*node+1;
    }
    static private int rch(int node){
      return 2*node+2;
    }
    public String toString(){
      return Arrays.toString(a);
    }
  }
  static boolean next_permutation(int[] per){
    if(per.length<2){
      return false;
    }
    int i;
    for(i=per.length-1;i>0;i--){
      if(per[i-1]<per[i]){
        break;
      }
    }
    if(0<i){
      i--;
      int tmp;
      int j;
      for(j=per.length-1;j>i;j--){
        if(per[j]>per[i]){
          break;
        }
      }
      //swap(i,j)
      tmp=per[i];
      per[i]=per[j];
      per[j]=tmp;
      for(int k=i+1;k<(i+1+per.length)/2;k++){
        //swap(k,per.length-k+i)
        tmp=per[k];
        per[k]=per[per.length-k+i];
        per[per.length-k+i]=tmp;
      }
      return true;
    }
    int tmp;
    for(int k=0;k<per.length;k++){
      //swap(k,per.length-k-1)
      tmp=per[k];  
      per[k]=per[per.length-k-1];
      per[per.length-k-1]=tmp;
    }
    return false;
  }
  static boolean next_bits(boolean[] b){
    for(int i=0;i<b.length;i++){
      if(b[i]){
        b[i]=false;
      }else{
        b[i]=true;
        return true;
      }
    }
    return false;
  }
  static class Scnr{
    private final InputStream ins;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    Scnr(){
      this(System.in);
    }
    Scnr(InputStream in){
      ins = in;
    }
    private boolean hasNextByte(){
      if(ptr<buflen){
        return true;
      }else{
        ptr = 0;
        try{
          buflen = ins.read(buffer);
        }catch(IOException e){
          e.printStackTrace();
        }
        if(buflen<=0){
          return false;
        }
      }
      return true;
    }
    private int readByte(){
      if(hasNextByte()){
        return buffer[ptr++];
      }else{
        return -1;
      }
    }
    private static boolean isPrintableChar(int c){
      return 33<=c&&c<=126;
    }
    public boolean hasNext(){
      while(hasNextByte()&&!isPrintableChar(buffer[ptr])){
        ptr++;
      }
      return hasNextByte();
    }
    public String next(){
      return nextBuilder().toString();
    }
    public StringBuilder nextBuilder(){
      if(!hasNext()){
        throw new NoSuchElementException();
      }
      StringBuilder sb = new StringBuilder();
      int b = readByte();
      while(isPrintableChar(b)){
        sb.appendCodePoint(b);
        b = readByte();
      }
      return sb;
    }
    public double nextDouble(){
      return Double.parseDouble(next());
    }
    public long nextLong(){
      if(!hasNext()){
        throw new NoSuchElementException();
      }
      long n = 0;
      boolean minus = false;
      int b = readByte();
      if(b=='-'){
        minus=true;
        b=readByte();
      }
      if(b<'0'||'9'<b){
        throw new NumberFormatException();
      }
      while(true){
        if('0'<=b&&b<='9'){
          n*=10;
          n+=b-'0';
        }else if(b==-1||!isPrintableChar(b)){
          return minus?-n:n;
        }else{
          throw new NumberFormatException();
        }
        b=readByte();
      }
    }
    public int nextInt(){
      long nl=nextLong();
      if(nl<Integer.MIN_VALUE||Integer.MAX_VALUE<nl){
        throw new NumberFormatException();
      }
      return (int) nl;
    }
    public String[] nextA(int n){
      String[] a = new String[n];
      nextA(a,n);
      return a;
    }
    public void nextA(String[] a,int n){
      nextA(a,0,n);
    }
    public void nextA(String[] a,int off,int len){
      for(int i=off;i<off+len;i++){
        a[i]=next();
      }
    }
    public int[] nextAInt(int n){
      int[] a = new int[n];
      nextAInt(a,n);
      return a;
    }
    public void nextAInt(int[] a,int n){
      nextAInt(a,0,n);
    }
    public void nextAInt(int[] a,int off,int len){
      for(int i=off;i<off+len;i++){
        a[i] = nextInt();
      }
    }
    public long[] nextALong(int n){
      long[] a = new long[n];
      nextALong(a,n);
      return a;
    }
    public void nextALong(long[] a,int n){
      nextALong(a,0,n);
    }
    public void nextALong(long[] a,int off,int len){
      for(int i=off;i<off+len;i++){
        a[i] = nextLong();
      }
    }
    public double[] nextADouble(int n){
      double[] a = new double[n];
      nextADouble(a,n);
      return a;
    }
    public void nextADouble(double[] a,int n){
      nextADouble(a,0,n);
    }
    public void nextADouble(double[] a,int off,int len){
      for(int i=off;i<off+len;i++){
        a[i] = nextDouble();
      }
    }
    public List<Integer> nextLInt(int n){
      List<Integer> l = new ArrayList<>(n);
      for(int i=0;i<n;i++){
        l.add(sc.nextInt());
      }
      return l;
    }
    public List<Long> nextLLong(int n){
      List<Long> l = new ArrayList<>(n);
      for(int i=0;i<n;i++){
        l.add(sc.nextLong());
      }
      return l;
    }
    public List<Double> nextLDouble(int n){
      List<Double> l = new ArrayList<>(n);
      for(int i=0;i<n;i++){
        l.add(sc.nextDouble());
      }
      return l;
    }
  }
  
  static Scnr sc = new Scnr();
  static PrintWriter out = new PrintWriter(System.out);
  public static void main(String args[]){
    //for(int i=sc.nextInt();i>0;i--)
    solve();
    out.flush();
  }
  static void solve(){
    int n=sc.nextInt();
    int m=sc.nextInt();
    long sum=0;
    int[] a=sc.nextAInt(m);
    for(int i=0;i<m;i++){
      sum=sum+a[i];
    }
    out.println(Math.max(n-sum,-1));
  }
}