#include<cstdio>
#include<vector>


using namespace std;


struct stage_t
{
	int l, s;
};


int min_u(int& m, int v)
{
	if(m>v)
	{
		m=v;
		return 1;
	}
	return 0;
}


int main(void)
{
	vector<stage_t> stage;
	vector<int> check;
	int n, i, sum, lose;

	while(scanf("%d", &n)==1)
	{
		stage.clear();
		stage.resize(n+1);
		check.clear();
		check.resize(n+1);

		sum=0;
		for(i=1;i<=n;i++)
		{
			scanf("%d%d", &stage[i].l, &stage[i].s);
			sum+=stage[i].l;
		}

		lose=0;
		for(i=1;i<=n;i++)
		{
			int t, ls, min;

			if(check[i]) continue;
			for(t=stage[i].s;!check[t];t=stage[t].s)
			{
				check[t]=i;
			}
			if(check[t]!=i) continue;

			ls=t;
			min=stage[ls].l;
			for(t=stage[ls].s;t!=ls;t=stage[t].s)
			{
				min_u(min, stage[t].l);
			}
			lose+=min;
		}

		printf("%.1f\n", 0.5*(sum+lose));
	}

	return 0;
}