Converting an integer from base n to base m?

int value[15]={1,0,2,3,4,5,6,7,8,9,10,11,12,13,14};
int base =15;
unsigned long long sum=0;
for (int i=0;i<15;i++)
{
    sum+=(pow(base,i)*value[14-i]);
}
cout << sum << endl; 

Can someone please write a program for converting any number from base n to base m ??? I already know how to convert a number from anu base to 10, but this program, I don't know how to make. Can someone help ?