#include <iostream>
#include <cmath>
using namespace std;

int main() {
	int b1, b2, b3, need, pas;
	cin >> b1 >> b2 >> b3;
	need = (b1 + b2 + b3) / 3;
	if ((b1 + b2 + b3) % 3!=0) {
		cout << "IMPOSSIBLE";
		return 0;
	}
	else cout << (abs(b1 - need) + abs(b2 - need) + abs(b3 - need))/2;
	return 0;
}