#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
ld calc(int av, int total, int move)
{
if(move == 1001)
return 0;
return ((ld)av / total) * move + (1 - ((ld)av / total)) * calc(av, total, move + 1);
}
int main()
{
ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
cout << calc(2, 3, 1);
return 0;
}
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