@[panrong](/user/1044970)
```cpp
#include <bits/stdc++.h>
using namespace std;
int main(){
long long a,b,c;
cin>>a>>b>>c;
long long d=b-a;
long long e=a+(c-1)*d;
cout<<(a+e)*c/2;
return 0;
}
```
by wzj0829 @ 2024-01-03 18:23:20
@[wzj0829](/user/1028403)
已关thank you
by panrong @ 2024-01-03 18:25:38