```c
#include<iostream>
using namespace std;
int main()
{
long long n,a,b
cin>>a>>b>>n;
cout<<(a+(n-1)*(b-a)+a)*n/2;
return 0;
}
by 2345A @ 2022-12-15 14:31:56
打错了要加一个;号
by 2345A @ 2022-12-15 14:34:12
@[baiqian](/user/565070) 为啥不能用double
by written_by_chiyu @ 2022-12-15 14:46:07
@[written_by_chiyu](/user/897433) 因为它是小数
by 2345A @ 2022-12-15 14:47:48
@[written_by_chiyu](/user/897433) 不比longlong大
by 2345A @ 2022-12-15 14:48:41