WA了6个。。不知道是咋回事

#include <bits/stdc++.h>
using namespace std;
char a[1010][1010], b[2020][2020];
int n, m, k;

int main() {
	cin >> n >> m >> k;
	bool f = 0;
	for (int i = 1; i <= n; i++)
		for (int j = 1; j <= m; j++) {
			cin >> a[i][j];
			if (a[i][j] != a[1][1])
				f = 1;
		}

	if (f == 0) {
		for (int i = 1; i <= n * k; i++) {
			for (int j = 1; j <= m * k; j++)
				cout << a[1][1];
			cout << endl;
		}
		return 0;
	}

	for (int i = 1; i <= n ; i++)
		for (int j = 1; j <= m ; j++) {//将a[i][j]赋值
			for (int x = i - 1; x <= i + k; x++) //矩阵运算。
				for (int y = j - 1; y <= j + k; y++)
					b[i + x][j + y] = a[i][j];

		}

	for (int i = 1; i <= n * k; i++) {
		for (int j = 1; j <= m * k; j++)
			cout << b[i][j];
		cout << endl;
	}//输出完毕
	return 0;
}