#include <bits/stdc++.h> using namespace std;

int main(){ int a1,a2,n,cha,an; cin>>a1>>a2>>n; cha=a2-a1; an=a1+(n-1)*cha; cout<<an; return 0; } 简便方法来啦！

#include <stdio.h>
#include <math.h>
int main()
{
    double x1,x2,y1,y2,x3,y3;
    double a,b,c,p;
    scanf("%lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3);
    a=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
    b=sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3));
    c=sqrt((x2-x3)*(x2-x3)+(y2-y3)*(y2-y3));
    p=1.0/2.0*(a+b+c);
    printf("%.2lf\n",sqrt(p*(p-a)*(p-b)*(p-c)));
    return 0;
}