hacker rank street light

#include<bits/stdc++.h>
using namespace std;
int main(){
    int n, q;
    cin>>q>>n;
    //use a scanline algorithm
    // Dark -- if element is --- a : zero, b: a[i] > 0
    //light ---if element is 1 i.e a[i] == 1 means light 
    int a[n + 1] = {0};// marks all the elements are dark(a[i] is 0)
    //performe quries
    for(int i = 0 ; i < q; i++){
        int l, r;
        cin>>l>>r;
        //use a sanline algorithm
        if( l - r >= 0)
          a[l -r]++; //a[l -r] += 1;
        else
      a[0]++;    // a[0] += 1;
        if(l + r + 1 <= n){
             a[l + r + 1]--; // a[l + r] -= 1;

        }

    }
    //find a prefix sum
    for(int i = 1; i<=n; i++)
    a[i] +=a[i -1];

    int cnt , res = -9999999;
    //traverse every elements of array if a[i]>=0 then count i.e we find a dark
    //if a[i] == 1 i.e light
    
    for(int i= 0; i <= n; i++){
            
            cnt = 0;
            //if a[i] >=0 and a[i] is not equal to 1 then count++
            while( a[i] != 1 && i<= n){
                i++;
                cnt++;
            }
            //store a max continious dark
            res = max(res , cnt);

    }
    cout<<res<<endl;
}