fprintf

#include <stdio.h>
// #include <stdlib.h>
int main() {
    FILE *S1, *S2;
    int x = 3, y;
    // system("chcp 1251");
    // system("cls");
    // printf("Введите число: ");
    // scanf("%d", &x);
    S1 = fopen("S1.txt", "w");
    fprintf(S1, "%d", x);
    fclose(S1);
    S1 = fopen("S1.txt", "r");
    S2 = fopen("S2.txt", "w");
    fscanf(S1, "%d", &y);
    y *= 3;
    fclose(S1);
    fprintf(S2, "%d\n", y);
    fclose(S2);
    return 0;
}