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

int le[51], ri[51];
bool mark[51];

bool is_acceptable(int x, int y)
{
    if (le[x] <= y && y <= ri[x])
    {
        if (le[y] <= x && x <= ri[y])
            return true;
    }
    return false;
}

int main()
{
    memset(mark, false, sizeof(mark));
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)
        cin >> le[i] >> ri[i];
    
    int ans = 0;
    for (int i = 0; i < n; i++)
    {
        if (mark[i])
            continue;
        // now for person i, we iterate through le[i] to ri[i] and check if they also want to become teammate with them
        for (int j = max(i + 1, le[i]); j <= ri[i]; j++)
        {
            if (mark[j])
                continue;
            if (is_acceptable(i, j))
            {
                // now let's look for a third teammate
                for (int k = max(j + 1, le[j]); k <= ri[i]; k++)
                {
                    if (mark[k])
                        continue;
                    if (is_acceptable(i, k) && is_acceptable(j, k))
                    {
                        mark[i] = mark[j] = mark[k] = true;
                        ans++;
                        break;
                    }
                    if (mark[k])
                        break;
                }
            }
            if (mark[j])
                break;
        }
        if (mark[i])
            continue;
    }
    cout << ans << endl;
}