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#include <bits/stdc++.h>
using namespace std;
//正确的代码，消除了浮点数的误差
//方法：将所有数字乘10，然后累加。
int main() {
int n;
double s = 0;
cin >> n;
for (int i = 1; i <= 10 * n - 1; i++) s += i;
cout << s / 10;
return 0;
}
