Help me in solving DSAAGP14 problem

help
,
1

My issue

unable to print for first and last query

My code

#include <bits/stdc++.h>
using namespace std;

int main() {
	// your code goes here
	int n;
	cin>>n;
	int arr[n];
	for(int i=0;i<n;i++){
	    cin>>arr[i];
	}
	
	
	int prefixarr[n];
	 prefixarr[0]=arr[0];
	for(int i=1;i<n;i++){
	    prefixarr[i]=prefixarr[i-1]+arr[i];
	}
	
	int m;
	cin>>m;
	while(m--){
	    
	    int a,b;
	    cin>>a>>b;
	    int sum;
	    
	    
	        prefixarr[0]=arr[0];
	        
	        sum=prefixarr[b-1]-prefixarr[a-1-1];
	        
	    cout<<sum<<endl;
	}

}

Learning course: Data Structures and Algorithms
Problem Link: https://www.codechef.com/learn/course/dsa-detailed/DSAAG03/problems/DSAAGP14