Trouble sort

#include<stdio.h>
#include<algorithm>
#include<vector>
using namespace std;
int main()
{
    int T;
    scanf("%d",&T);
    for(int t = 1 ; t <= T ; t ++){
        int n;
        scanf("%d",&n);
        vector<int> A;
        vector<int> B;
        for(int i = 1 ; i <= n ; i ++){
            int x;
            scanf("%d",&x);
            if(i % 2 == 1){
                A.push_back(x);
            }else{
                B.push_back(x);
            }
        }
        sort(A.begin(),A.end());
        sort(B.begin(),B.end());
        int ans = n;
        int idx = 0;
        for(int i = 0 ; i < A.size() ; i ++){
            int front = -1;
            if(i-1>=0)front = B[i-1];
            int back = 1e9+1;
            if(i<B.size()){
                back = B[i];
            }
            if(!(front <= A[i] && A[i] <= back)){
                if(idx < ans)ans = idx;
                break;
            }
            idx+=2;
        }
        idx = 1;
        for(int i = 0 ; i < B.size() ; i++){
            int front = -1;
            front = A[i];
            int back = 1e9+1;
            if(i+1 < A.size())back = A[i+1];
            if(!(front <= B[i] && B[i] <= back)){
                if(idx < ans)ans = idx;
                break;
            }
            idx+=2;
        }
        if(ans == n){
            printf("Case #%d: OK\n",t);
        }else{
            printf("Case #%d: %d\n",t,ans);
        }
    }
    return 0;
}