View Compilation Error

0_0_18669533_32301.cpp: In function 'int main()':
0_0_18669533_32301.cpp:16:32: error: 'd' was not declared in this scope
             if(2*f[j]+d(2.0,i-j)-1<min)  //总步数：2*F[x]+2^(n-x)-1 
                                ^