Oh! Now I see... I thought I had the freedom of skipping some bulbs, seems that is not the case :P

I can not understand where I am stuck. I am getting 1 and 2 as answer to the sample test case in your above comment, But I'am getting WA in 9th test case:

//
// Created by sourabh on 15/2/18.
//
#include <bits/stdc++.h>
using namespace std;

long long int binarySearch(long long int start, long long int dis, long long int pwr) {
    long long int end = 1;
    while(start > end) {
        int mid = end + (start - end) / 2 + 1;
        if(dis - mid >= pwr)
            end = mid;
        else
            start = mid - 1;
    }
    if(dis - start > pwr)
        return start + 1;
    return start;
}

int main() {
    long long int n, resl = INT_MAX;
    cin >> n;
    pair<long long int, long long int> ar[n];
    long long int prefix[n+1] = {0};

    for(auto &i : ar) {
        cin >> i.first;
        cin >> i.second;
    }

    for(long long int i = 2; i <= n; ++i) {
        long long int pos = binarySearch(i - 1, ar[i-1].first, ar[i-1].second);
        prefix[i] = i - pos + prefix[pos-1];
    }

    for(long long int i = n; i >= 1; --i)
        resl = min(resl, n - i + prefix[i]);
    cout << resl;
}