public static int pylons(int k, List<Integer> arr) {
        int ans = 0, n = arr.size();
        for(int i = 0; i < n; i++){
            
            // Power plant should be built on buildIdx
            int buildIdx = Math.min(i + (k - 1), n - 1);
            
            // Keep looking for a power plant
            while(buildIdx >= 0 && buildIdx < n && arr.get(buildIdx) == 0)
                buildIdx--;
                
            if(buildIdx >= 0 && buildIdx < n && arr.get(buildIdx) == 1 && Math.abs(buildIdx - i) < k){
                ans++; // Build here
                i = buildIdx + (k - 1);
            }
            else
                return -1;
        }
        return ans;
    }