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13 lines
223 B
C++
13 lines
223 B
C++
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// R38704401
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#include <bits/stdc++.h>
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using namespace std;
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int main() {
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long long a[2], n;
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scanf("%lld%lld%lld", &a[0], &a[1], &n);
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cout << n * a[0] + n * (n - 1) * (a[1] - a[0]) / 2 << endl;
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return 0;
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}
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