#include <bits/stdc++.h>
using namespace std;
int getMissingno(int a[],int n){
int N=n+1;
int total=N*(N+1)/2;
for(int i=0;i<n;i++)
total-=a[i];
return total;
}
int main() {
// your code goes here
int arr[]={1,2,4,5};
int N=sizeof(arr)/sizeof(arr[0]);
int miss=getMissingno( arr, N);
cout<<miss;
return 0;
}
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