等差数列求和，怎么改啊

#include<bits/stdc++.h>
using namespace std;
int a,b,c,sum;
int main(){
    cin>>a>>b>>c;//scanf("%d %d %d",&a,&b,&c);
    for(int i=a;i<=b;i+=c){
        sum+=i;
    }
    cout<<sum;//printf("%d",sum);
    return 0;
}