Revise paste Untitled

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

int main(int argc, char *argv[]) {
    FILE *i1 = fopen(argv[1], "r");
    FILE *i2 = fopen(argv[2], "r");
    FILE *o = fopen(argv[3], "w");
    
    if(i1 == NULL || i2 == NULL) {
        printf("GRESHKA");
        return 0;
    }
    int bukvi1[26], bukvi2[26];
    char niza1[1001], niza2[1001];
    while(fgets(niza1, 500, i1)) {
        fgets(niza2, 500, i2);
        for(int i = 0; i < 26; i++) {
            bukvi1[i] = 0;
            bukvi2[i] = 0;
        }
        for(int i = 0; i < strlen(niza1); i++) {
            if(isalpha(niza1[i])) {
                niza1[i] = tolower(niza1[i]);
                bukvi1[niza1[i] - 'a'] = 1;
            }
        }
        for(int i = 0; i < strlen(niza2); i++) {
            if(isalpha(niza2[i])) {
                niza2[i] = tolower(niza2[i]);
                bukvi2[niza2[i] - 'a'] = 1;
            }
        }
        for(int i = 0; i < 26; i++) {
            if(bukvi1[i] == 1 && bukvi2[i] == 1) {
                fprintf(o, "%c,", (char)(i + 'a'));
            }
        }
        fprintf(o, "\n");
    }
    fclose(o);
    FILE *i3 = fopen(argv[3], "r");
    while(fgets(niza1, 500, i3)) {
        printf("%s\n", niza1);
    }
    return 0;
}
/*
 
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 removed from the definition, a phenomenon known as the AI
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 **/