```cpp
#include<bits/stdc++.h>
using namespace std;
long long a,b,c;
int main()
{
cin>>a>>b>>c;
cout<<(b+(a-1)*(c-a))*a/2;
return 0;
}
```
by TLHyyds @ 2022-02-28 17:18:34
@[TLHyyds](/user/576742) 不确定是不是公式的问题~~因为我太蒻~~,我的代码:
```cpp
#include <bits/stdc++.h>
using namespace std;
int main (){
long long a,b,n;
cin >> a >> b >> n;
cout << (a+(n-1)*(b-a)+a)*n/2;
return 0;
}//可以参考一下
```
by chickencxk @ 2022-03-17 12:36:09
谢谢
by TLHyyds @ 2022-03-17 14:35:14