语句顺序问题

#include <stdio.h>
int main(){
    int n,k;
    scanf("%d %d",&n,&k);
    int c = n/k;
    long int ttl = n*(n+1)/2;
    double s1 = k*(c+1)/2;
    double s2 = (ttl-c*s1)/(n-c);
    printf("%.1f %.1f",s1,s2);  
}

double s1 = k*(c+1)/2; 这一行，如果改为
double s1 = (c+1)/2*k; 则错误。为啥？