#include <stdio.h>

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