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)*(-1);
        cout<<ans<<endl;
    }
    while(1);
}