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22 lines
549 B
C++
22 lines
549 B
C++
#include <bits/stdc++.h>
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using namespace std;
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int n;
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double a[100005], l1[100005], l2[100005], l3[100005], ans[100005];
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int main() {
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cin >> n;
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for (int i = 1; i <= n; i++) {
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cin >> a[i];
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}
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for (int i = 1; i <= n; i++) {
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l1[i] = a[i] * (l1[i - 1] + 1);
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l2[i] = a[i] * (l2[i - 1] + 2 * l1[i - 1] + 1);
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l3[i] = a[i] * (l3[i - 1] + 3 * l2[i - 1] + 3 * l1[i - 1] + 1);
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ans[i] = ans[i - 1] + l3[i] - a[i] * l3[i - 1];
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}
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cout << fixed << setprecision(1) << ans[n] << endl;
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return 0;
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}
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