#include<bits/stdc++.h>
using namespace std;
int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    freopen("animal.in", "r", stdin);
    freopen("animal.out", "w", stdout);
    int a,b,c,d=0;
    cin>>a>>b>>c;
    if(b>a){
        c-=b-a;
        a=b;
    }
    while(c>=0){
        d+=a-b+(c-a+b)/2;
        c--;
    }
    cout<<d;
}