#include <bits/stdc++.h>

using namespace std;

int main(){
    int n, m, k, v;
    cin >> n >> m >> k >> v;

    if(k - n <= 0){
        if(v-(m-n) > 0){
            cout << v-(m-n) << endl;
        }
        else{
            cout << 0 << endl;
        }
    }
    else{
        if((k+v)-(n+m) > 0){
            cout << (k+v)-(n+m) << endl;
        }
        else{
            cout << 0 << endl;
        }
    }
}