Suppose the input for a problem is positive integer N and answer happens to be 1/N. Here is what happens for different constraints on N. You decide which version is the hardest.

1.

`N <= 10^9`

Everyone does math and all solutions look like

```
cout << 1.0 / N
```

2.

`N <= 10^6`

You see solutions from above but some contestants decide to use iterative approach, something like

```
double ans = 1.0;
for (int i = 2; i <= N; i++) ans = ans * (i - 1) / i;
```

3.

`N <= 10^5`

Mostly solutions from above, but some contestants use recursion (maybe with memoization)

```
double f(int N) {
if (N == 1) return 1.0;
return f(N - 1) * (N - 1) / N;
}
```

4.

`N <= 500`

Solutions from above, but some contestants go with O(N^3) dynamic programming instead

5.

`N <= 109`

Contestants think there is a typo and go with first solution

6.

`N <= 100`

Some contestants realize that you can precompute all answers

```
const double ans[100]={1.0, 0.5, 0.333333333, ... }
```

7.

`N <= 47`

Intended solution from Ukrainian author has O(N^4) complexity

8.

`N <= 36`

You see lot of solutions with meet-in-the middle approach

9.

`N <= 20`

You start to see a lot of bitwise operations in codes that can mean dynamic programming on subsets

10.

`N <= 10`

Some C++ codes use next_permutation(), Java coders having harder time

LOL

N≤ 5. Long contest. THE HARDEST PROBLEM IN THE CONTEST!N≤ 50. 70% of all solutions fall in challenge phase, 50% of them in the first 30 seconds.N≤ 10^{6}, Hungary: TL is too low, no chance of reading the input (CEOI, practice session).LOL hahaha

sometimes it produces trap foro contestants.

`N <= 18`

and they starts to think about bitmasks though it is a sorting problem xD