#include <iostream>
#include <vector>

int main()
{
    int n;
    std::cin >> n;
    std::vector<int> a(n);
    std::vector<int> b(n);
    
    for(int i=0; i<n; i++)
    {
        std::cin >> a[i] >> b[i];
        a[i]--;
        b[i]--;
    }
    
    int teams = 0;
    std::vector<bool> used(n);

    for(int i=0; i<n; i++)
        for(int j=i+1; j<n; j++)
            for(int k=j+1; k<n; k++)
            {
                if(used[i] || used[j] || used[k])
                    continue;
                if(b[i] < k || b[i] < j || b[j] < k) continue;
                if(a[j] > i || a[k] > i || a[k] > j) continue;
                teams++;
                used[i] = true;
                used[j] = true;
                used[k] = true;
            }
    std::cout << teams << std::endl;
}