```cpp
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 1e6+7;
int main()
{
ll a1,a2,n,sum=0,a3;
scanf("%lld %lld %lld",&a1,&a2,&n);
sum += (a1 + a2);
a3 = (a2 + (a2 - a1));
for (ll i = 3; i <= n; i++)
{
sum += a3;
a3 += (a2 - a1);
}
cout << sum;
return 0;
}
```
by xxk2006 @ 2023-08-23 22:08:17
"a2"与"a1"打反了，应该是"a2-a1"
```cpp
#include<iostream>
using namespace std;
int main()
{
long long f1,f2,c,len,ans=0;
cin>>f1>>f2>>len;
c=f2-f1;
for(int i=0;i<len;i++)
{
ans+=f1;
f1+=c;
}
cout<<ans<<endl;
return 0;
}
```
by AZYDLL @ 2023-08-23 22:55:33