Missing integer in arithmetic sequence

gistfile1.c

#include <stdio.h>

int main(void) {
  int n, e, i = 0;
  scanf("%d\n", &n);
  scanf("%d", &e);
  int sum = e, first = e, last = e, len = 1;
  int delta;
  while (++i < n) {
    scanf(" %d", &e);
    sum += e;
    len++;
    int tmp = e - last;
    last = e;
    if (i > 1 && tmp != delta) 
      break;
    delta = tmp;
  }
  int seqSum = (first + last) * (len + 1) / 2;
  printf("%d\n", seqSum - sum);
}