HDU 2685 I won't tell you this is about number theory

#include <stdio.h>

int gcd(int a, int b)
{
return !b ? a : gcd(b, a%b);
}

int mod_pow(int a, int x, int mod)
{
int tmp = a;
int ret = 1;
while(x) {
if (x & 1) {
ret = ret * tmp % mod;
}
tmp = tmp * tmp % mod;
x >>= 1;
}

return ret;
}

int main()
{
int t, a, m, n, k;
scanf("%d", &t);
while (t--) {
scanf("%d %d %d %d", &a, &m, &n, &k);
int d = gcd(m, n);
int ans = mod_pow(a, d, k);
printf("%d\n", (ans - 1 + k) % k);
}

return 0;
}