WHY THIS SOLUTION WAS NOT GETTING ACCEPTED AT STARTERS 96 DIV 4 (D,REMOVE MULTIPLES);

general
1

// Problem Link → REMOVEMUL Problem - CodeChef

int n,k;
cin>>n>>k;
int a[k];
ll sum=0;
for(int i=0;i<k;i++){
cin>>a[i];
sum+=a[i];
}

cout << (n*(n+1))/2-(sum) << endl;
2

N range is from 1 to 10^9 and Q is subset of 1 to N. so use this
long int N, K;
long int Q[N];