#include<stdio.h>

void run(void){
  int a,b,c,d;
  scanf("%d%d%d%d",&a,&b,&c,&d);
  int i,j;
  int ans=0;
  for(i=a;i<=b;i++){
    for(j=c;j<=d;j++){
      if(i!=j) ans++;
    }
  }
  printf("%d\n",ans);
}

int main(void){
  run();
  return 0;
}