swapping 1st and last digits in a number

#include <stdio.h>
#include <math.h>
#define ULL unsigned long long

ULL findNoOfDigits(ULL n){//counting digits
    ULL digits = 1;
    for(ULL i=10;n%i!=n;i*=10,digits++);
    return digits;
}

int main(void) {
    ULL n,lp,rp,mul;
    scanf("%llu",&n);
    mul = (ULL)pow(10,findNoOfDigits(n)-1);
    rp = n%10;
    lp = ((n-n%mul)/mul);
    n = (n%(lp*mul)+rp*mul) + lp-rp;
    printf("\n%llu",n);
    return 0;
}