#include <bits/stdc++.h>
using namespace std;
long long unsigned int countDivisors(long long unsigned int n) {
  long long unsigned int cnt = 0;
  for (long long unsigned int i = 1; i <= sqrt(n); i++) {
    if (n % i == 0) {
      if (n / i == i)
        ++cnt;
      else
        cnt += 2;
    }
  }
  return cnt;
}
int findFirst(const std::vector<int> &array, int key, int low, int high) {
  int ans = -1;
  while (low <= high) {
    int mid = low + (high - low) / 2;
    if (key == array[mid]) {
      ans = mid;
      high = mid - 1;
    } else if (key > array[mid])
      low = mid + 1;
    else
      high = mid - 1;
  }
  return ans;
}
int findLast(const std::vector<int> &array, int key, int low, int high) {
  int ans = -1;
  while (low <= high) {
    int mid = low + (high - low) / 2;
    if (key == array[mid]) {
      ans = mid;
      low = mid + 1;
    } else if (key > array[mid])
      low = mid + 1;
    else
      high = mid - 1;
  }
  return ans;
}
int d, x, y;
void extendedEuclid(int a, int b) {
  if (b == 0) {
    d = 1;
    x = 1;
    y = 0;
  } else {
    extendedEuclid(b, a % b);
    int temp = x;
    x = y;
    y = temp - (a / b) * y;
  }
}
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  int a, b, c, n;
  cin >> a >> b >> c >> n;
  if (n >= 1 + a + b - c && c <= a && c <= b) {
    int ans = n - (a + b - c);
    cout << ans << endl;
  } else {
    cout << "-1" << endl;
  }
  return 0;
}
