A program to show key exchange using Diffie-Hallman key exchange

The Diffie-Hellman algorithm is being used to establish a shared secret that can be used for secret communications while exchanging data over a public network using the elliptic curve to generate points and get the secret key using the parameters. 

#include<stdio.h>
#include<math.h>
long long int power(long long int a, long long int b,
                                    long long int P)
{
   if (b == 1)
       return a;
   else
       return (((long long int)pow(a, b)) % P);
}
int main()
{
   long long int P, G, x, a, y, b, ka, kb;
   printf("Enter the value of P: ");
   scanf("%lld", &P);
   printf("Enter the value of G: ");
   scanf("%lld", &G);
   printf("Enter the value of private key for A, a: ");
   scanf("%lld", &a);
   x = power(G, a, P);
   printf("Enter the value of private key for B, b: ");
   scanf("%lld", &b);
   y = power(G, b, P);
   ka = power(y, a, P);
   kb = power(x, b, P);
   
   printf("Secret key for the Alice is : %lld\n", ka);
   printf("Secret Key for the Bob is : %lld\n", kb);
   
   return 0;
}