POJ 2521

 #include <iostream>
 #include <stdio.h>
 using namespace std;

 int main()
 {
     //freopen("acm.acm","r",stdin);
     int a;
     int b;
     int c;
     int d;
     int ans;
     do
     {
         cin>>a>>b>>c>>d;
         if(!a&&!b&&!c&&!d)
         {
             break;
         }
         ans = (b - c - a)*(-);
         cout<<ans<<endl;
     }
     while();
 }