为什么输入对，但却上交编译错

#include<bits/stdc++.h> using namespace std; int i,j,n,a1,a2,b1,b2,c1,c2,pen1,pen2,pen3,x[3]; int compare(int a1,int a2,int b1,int b2,int c1,int c2) { while(pen1<=n) { pen1+=a1; x[0]+=a2; } while(pen2<=n) { pen2+=b1; x[1]+=b2; } while(pen3<=n) { pen3+=c1; x[2]+=c2; } return x[0],x[1],x[2]; } int main() { cin>>n>>a1>>a2>>b1>>b2>>c1>>c2; compare(a1,a2,b1,b2,c1,c2); sort(x,x+3); cout<<x[0]; return 0; }