zadaca studenti strukturi

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
struct student{
char ime[20];
char prezime[40];
int indeks;
int zad1,zad2,zad3,zad4;
};

typedef struct student student;

void polni(student *s){
scanf("%s",&s->ime);
scanf("%s",&s->prezime);
scanf("%d",&s->indeks);
scanf("%d %d %d %d",&s->zad1,&s->zad2,&s->zad3,&s->zad4);
}
int vkupnobod(student s){
return s.zad1+s.zad2+s.zad3+s.zad4;
}
void sredi(char *s){
*s = toupper(*s);
  while (*(++s) != '\0')
  *s = tolower(*s);
}
void sort(student a[], int n) {
  int i, j;
  student s;
  for (i = 0; i < n; i++)
  for (j = 0; j < n - i - 1; j++)
  if (vkupnobod(a[j]) < vkupnobod(a[j+1])) {
  s = a[j];
  a[j] = a[j + 1];
  a[j + 1] = s;
  }
}
int main()
{
int i;
int n;
student a[50];
scanf("%d",&n);
for(i=0;i<n;i++)
{
    polni(&a[i]);
    sredi(a[i].prezime);
}
sort(a,n);

  for (i = 0; i < n; i++) {
  printf("%d. %s %s %d %d\n", i+1, a[i].ime, a[i].prezime, a[i].indeks, vkupnobod(a[i]));
}

    return 0;
}