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 C++ (GCC 9.2.1) (judge verdict: Accepted)

//AnkitCode99 here....
#include<bits/stdc++.h>
#define endl "\n"
typedef long long int ll;
#define MOD 1000000007
#define mp make_pair
#define pll pair<ll,ll>
#define pb push_back
//max xor btw range of two numbers..
#define max_XOR(a,b) (1 << int(log2(a ^ b) + 1)) - 1
#define vl vector<ll>
#define inf ll(1e18)
#define all(v) v.begin(),v.end()
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define rep(i,a,b) for(long long int i=a;i<b;i++)
#define nrep(i,a,b) for(long long int i=a;i>=b;i--)
#define ceil(a,b) a/b+((a%b ==0 )?0:1)
using namespace std;

const ll sz=100005;
const ll szz=1000006;

int main()
{
    fast;
    #ifndef ONLINE_JUDGE
    freopen("input.txt","r",stdin);
    freopen("op.txt","w",stdout);
    #endif

    ll n,m;
    cin>>n>>m;
    // ll ar[m];
    ll sum=0;
    rep(i,0,m)
    {
        ll ele;
        cin>>ele;
        sum+=ele;
    }
    if(sum>n)
    {
        cout<<"-1";
    }
    else
    {
        cout<<n-sum;
    }
    
}//Goodbye...