In this problem you are given "x" amount of money in currency "s" and you want to maximise the amount in "m" steps to currency "f". cost matrix is given for transfer rate. Sounds similar to finding number of ways to reach from one node to another in k steps. So the answer to problem is (s,f) value of mth power of given cost matrix (exchange rate matrix). we need to modify matrix multiplication accordingly to get maximum cost.

matrix matmul(matrix &a,matrix &b)
	{
	  int n=a.size();
	  ll temp;
	  matrix ans(n,vector<ll>(n));
	  for(int i=0;i<n;i++)
	  {
	    for(int j=0;j<n;j++)
	    {
	      temp=0;
	      for(int k=0;k<n;k++)
	      {
	        temp=max(temp,a[i][k]*b[k][j]) %mod; // this is problem
	      }
	      ans[i][j]=temp;
	    }
	  }
	  return ans;
	}

But we want ans%mod , this mod will ruin my multiplication of matrix. I will not get maximum. Editorial suggest to use power of primes , can someone please explain that.
Problem Link