P1595 信封问题

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#include<cstdio>
#define int long long
#define re register
const int MAXn = 20;

int n, a[MAXn + 10];
signed main() {
    a[1] = 0; a[2] = 1;
    scanf("%lld", &n);
    for (re int i = 3; i <= n; ++i) {
        a[i] = (i - 1) * (a[i - 2] + a[i - 1]);
    }
    printf("%lld\n", a[n]);
}