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