Replace Bit Field in a 32-bit Register

#include <stdio.h>
#include <stdint.h>

uint32_t replace_field(uint32_t reg, uint32_t val, uint8_t pos, uint8_t len) {
    // Your code here
    //clear all bits except the choosed one's
    uint32_t mask =  (((1<<len) - 1)) << pos; //11 & 10
    reg &= ~mask;
    reg |= ((val << pos) & mask);
    // reg &=  (mask << pos); 

    return reg;
}

int main() {
    uint32_t reg, val;
    uint8_t pos, len;
    scanf("%u %u %hhu %hhu", &reg, &val, &pos, &len);
    printf("%u", replace_field(reg, val, pos, len));
    return 0;
}