设第一层中间两数依次为x, y，则

          c
    a+2x+y  x+2y+b
   a+x   x+y    y+b
  a    x     y     b

所以c = a+2x+y + x+2y+b
所以? = x+y = (c-a-b)/3

#include<iostream>
using namespace std;
int main(){
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    int a, b, c;
    cin >> a >> b >> c;
    cout << (c-a-b) / 3;
    return 0;
}

#include <iostream>
using namespace std;
int main()
{
int a = 0;
int b = 0;
int c = 0;
int x = 0;
cin >> a >> b >> c;
x = (c - a - b) / 3;
cout << x;
}