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- #include<bits/stdc++.h>
- using namespace std;
- int main() {
- int n;
- while (cin >> n) {
- double s = 0;
- for (int i = 1; i <= n; i += 2) {
- if (i % 4 == 1)
- s += 1.0 / i;
- else
- s -= 1.0 / i;
- }
- cout << fixed << 4 * s << endl;
- }
- return 0;
- }
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