include <bits/stdc++.h>

define ll long long

define ld long double

using namespace std; int main(){

ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
ll n, m, k, sum, ans = 0;
cin>>n>>m>>k;
sum = n+m+k;
if(sum%3 != 0){
    cout<<"IMPOSSIBLE";
    return 0;
}
sum/=3;
if(n>sum){
    ans+=(n-sum);
}
if(m>sum){
    ans+=(m-sum);
}
if(k>sum){
    ans+=(k-sum);
}
cout<<ans;

}//TECHNOBLADE NEVER DIE