#include "stdio.h"
#include <stdlib.h>

int main(int argc, char *argv[]) {
	if (argc < 4) {
		printf("Usage: %s file1.txt file2.txt output.txt\n", argv[0]);
		exit(1);
	}

	FILE *file1 = fopen(argv[1], "r");
	FILE *file2 = fopen(argv[2], "r");
	FILE *output = fopen(argv[3], "w");
	if (file1 == NULL||file2 == NULL||output == NULL) {
		printf("ERROR: Cannot open files\n");
		exit(1);
	}

	int file1Char;
	int file2Char;
	while ((file1Char = fgetc(file1)) != EOF && (file2Char = fgetc(file2)) != EOF) {
		if (file1Char == file2Char) {
			fputc(file1Char, output);
		} else {
			fputc(' ', output);
		}
	}

	fclose(file1);
	fclose(file2);
	fclose(output);
}