开unsigned long long吧
by tidongCrazy @ 2019-09-03 13:51:46
```
#include<bits/stdc++.h>
#define int unsigned long long
using namespace std;
signed main() {
int n,ch,a,b,sum=0;
scanf("%lld%lld%lld",&a,&b,&n);ch=b-a;sum+=(a+b);
for(int i=3;i<=n;i++)b+=ch,sum+=b;
printf("%lld",sum);
}
```
反正我是这么写的
by tidongCrazy @ 2019-09-03 13:51:59
@[提动Crazy](/space/show?uid=178519)
大佬，unsigned long long 似乎是无符号的，但是等差数列中必定可存在负数呀QAQwtrl!~~
by HOIer_9_42 @ 2019-09-04 12:44:58