diff --git a/src/main.c b/src/main.c
index e69de29..29fee8e 100644
--- a/src/main.c
+++ b/src/main.c
@@ -0,0 +1,128 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+// Function to convert a character to a digit
+int char_to_digit(char c) {
+    if (c >= '0' && c <= '9') {
+        return c - '0';
+    } else if (c >= 'A' && c <= 'F') {
+        return c - 'A' + 10;
+    } else if (c >= 'a' && c <= 'f') {
+        return c - 'a' + 10;
+    }
+    return -1; // Invalid character
+}
+
+// Function to convert a digit to a character
+char digit_to_char(int d) {
+    if (d >= 0 && d <= 9) {
+        return '0' + d;
+    } else if (d >= 10 && d <= 15) {
+        return 'A' + (d - 10);
+    }
+    return '\0'; // Invalid digit
+}
+
+// Function to convert a number from given base to decimal
+long long to_decimal(const char *num, int base) {
+    long long result = 0;
+    int len = strlen(num);
+    
+    for (int i = 0; i < len; i++) {
+        int digit = char_to_digit(num[i]);
+        if (digit == -1 || digit >= base) {
+            return -1; // Invalid digit for the given base
+        }
+        result = result * base + digit;
+    }
+    
+    return result;
+}
+
+// Function to convert a decimal number to given base
+void from_decimal(long long num, int base, char *result) {
+    if (num == 0) {
+        result[0] = '0';
+        result[1] = '\0';
+        return;
+    }
+    
+    char temp[65]; // Enough to hold a 64-bit number in binary
+    int index = 0;
+    
+    while (num > 0) {
+        temp[index++] = digit_to_char(num % base);
+        num /= base;
+    }
+    
+    // Reverse the string
+    for (int i = 0; i < index; i++) {
+        result[i] = temp[index - 1 - i];
+    }
+    result[index] = '\0';
+}
+
+// Function to validate if a number is valid in the given base
+int is_valid_number(const char *num, int base) {
+    int len = strlen(num);
+    if (len == 0) return 0;
+    
+    for (int i = 0; i < len; i++) {
+        int digit = char_to_digit(num[i]);
+        if (digit == -1 || digit >= base) {
+            return 0;
+        }
+    }
+    return 1;
+}
+
+int main(int argc, char *argv[]) {
+    // Check the number of arguments
+    if (argc != 4) {
+        fprintf(stderr, "Błąd: Nieprawidłowa liczba argumentów.\n");
+        fprintf(stderr, "Użycie: %s <liczba> <system_z> <system_na>\n", argv[0]);
+        return 1;
+    }
+    
+    // Parse the bases
+    int base_from, base_to;
+    if (sscanf(argv[2], "%d", &base_from) != 1 || 
+        sscanf(argv[3], "%d", &base_to) != 1) {
+        fprintf(stderr, "Błąd: Podstawy systemów muszą być liczbami całkowitymi.\n");
+        return 1;
+    }
+    
+    // Check if bases are in valid range
+    if (base_from < 2 || base_from > 16) {
+        fprintf(stderr, "Błąd: System źródłowy musi być w zakresie 2-16.\n");
+        return 1;
+    }
+    
+    if (base_to < 2 || base_to > 16) {
+        fprintf(stderr, "Błąd: System docelowy musi być w zakresie 2-16.\n");
+        return 1;
+    }
+    
+    // Check if the input number is valid in the source base
+    if (!is_valid_number(argv[1], base_from)) {
+        fprintf(stderr, "Błąd: Liczba '%s' jest nieprawidłowa w systemie o podstawie %d.\n", argv[1], base_from);
+        return 1;
+    }
+    
+    // Convert to decimal
+    long long decimal = to_decimal(argv[1], base_from);
+    if (decimal == -1) {
+        fprintf(stderr, "Błąd: Nie udało się przekonwertować liczby do systemu dziesiętnego.\n");
+        return 1;
+    }
+    
+    // Convert from decimal to target base
+    char result[65]; // Enough to hold a 64-bit number in binary
+    from_decimal(decimal, base_to, result);
+    
+    // Print the result
+    printf("%s\n", result);
+    
+    return 0;
+}