#include <stdio.h>

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