I was solving a problem "Sorting a three value sequence" USACO training gateway. I managed to solve the problem on my own. But in analysis section I found a solution given below, I was unable to understand whats the use of 2*(max(s12, s21)- min(s12, s21). plz explain me I am a newbie ?

#include <fstream>

using namespace std;

int min (int a, int b) { return a < b ? a : b; }
int max (int a, int b) { return a > b ? a : b; }

int main () {
    int s[1024];
    int n;
    int sc[4] = {0};
    
    ifstream fin("sort3.in");
    ofstream fout("sort3.out");
    fin>>n;
    for (int i = 0; i < n; i++) {
        fin>>s[i];
        sc[s[i]]++;
    }
    int s12 = 0, s13 = 0, s21 = 0, s31 = 0, s23 = 0, s32 = 0;
    for (int i = 0; i < sc[1]; i++){
        if (s[i] == 2) s12++;
        if (s[i] == 3) s13++;
    }
    
    for (int i = sc[1]; i < sc[1]+sc[2]; i++){
        if (s[i] == 1) s21++;
        if (s[i] == 3) s23++;
    }
    
    for (int i = sc[1]+sc[2]; i < sc[1]+sc[2]+sc[3]; i++){
        if (s[i] == 1) s31++;
        if (s[i] == 2) s32++;
    }
    
    fout<<min(s12, s21)+min(s13, s31)+min(s23, s32) +
					2*(max(s12, s21) - min(s12, s21))<<endl;
    return 0;
}