关于归并排序求逆序对数

利用归并排序求逆序对数，但是结果是错误的，帮忙看看哪里出错了

#include<stdio.h>
int index = 0;
int main() {
    void Merge(int a[], int a1[], int s, int m, int t);
    void MergeSort(int a[], int s, int t);
    int a[6];
    for (int i = 0; i < 6; i++) {
        scanf("%d", &a[i]);
    }
    printf("\n");
    MergeSort(a, 0, 5);
    printf("%d", index);
}
void Merge(int a[], int a1[], int s, int m, int t) {
    int i = s;
    int j = m + 1;
    int k = s;
    while (i <= m && j <= t) {
        if (a[i] <= a[j]) {
            a1[k] = a[i];
            k++;
            i++;
        }
        else {
            a1[k] = a[j];
            k++;
            j++;
            index += m - i + 1;
        }
    }
    while (i <= m) a1[k++] = a[i++];
    while (j <= t) a1[k++] = a[j++];
}
void MergeSort(int a[], int s, int t) {
    int a1[1000];
    int m;
    if(s < t) {
        m = (s + t) / 2;
        MergeSort(a, s, m);
        MergeSort(a, m + 1, t);
        Merge(a, a1, s, m, t);
    }
    for (int i = s; i < t; i++) {
        a[i] = a1[i];
    }
}

sort 中最后for循环应该i<=t吧