#include <stdio.h>

int main() {
  int a, b, c, d;
  scanf("%d %d %d %d", &a, &b, &c, &d);

  int cnt=0;
  for (int i=a; i<=b; i++) {
    for (int j=c; j<=d; j++) {
      if (i==j) cnt++;
    }
  }

  printf("%d\n", (b-a+1)*(d-c+1)-cnt);
  return 0;
}