c语言循环结构 指定范围内的特定值和

int cal(int left,int right,int target){
    if(left > right)return 0;
    int i = left,res = 0;
    for(;i <=right;++i){
        if(i%target == 0)res += i;
    }
    return res;
}

#include <stdio.h>

int main()
{
    int a,b,c;
    long long sum;
   scanf("%d %d %d",&a,&b,&c);
    long long i=c;
    for(;i<=b;){
        if(i>=a)sum+=i;
        i+=c;
        
    }
    printf("%lld",sum);
   return 0;
}