#include <stdio.h>

int main(void){
  int a,b,c,d,ans;
  scanf("%d %d %d %d",a,b,c,d);
  for(;b<=a;b++){
    for(;c<=d;c++){
      if(b!=c)ans++;
    }
  }
  printf("%d",ans);
  return 0;
}