#include <stdio.h>
#include <stdlib.h>

double func(double  x_p, int n_s, double *x , double *y);
double (*f_u) (double  x_p, int n, double *x , double *y);

int main(void)
{
double *x,*y, x_p, rez;
int n, i;
f_u = &func;
scanf("%d", &n);

x = (double*) malloc (sizeof(double)*n);
y = (double*) malloc (sizeof(double)*n);

for(i=0; i<n; i++){
scanf ("%lf %lf", &x[i], &y[i]);
}

printf("x:"); scanf("%lf", &x_p);

rez = f_u(x_p, n,  x , y);

printf ("%lf", rez);
return 0;
}

double func(double x_p, int n_s, double *x , double *y){
double  x1, x2, x3, y1, y2, y3, res, a0, a1, a2;
int n, i;

n = 0;

while (x_p > x[n]) n++;

x1 = x[n-1];
y1 = y[n-1];
x2 = x[n];
y2 = y[n];

if (n == 1){
    x3 = x[2];
    y3 = y[2];
}

if (n == n_s-1){
    x3 = x[n-2];
    y3 = y[n-2];
}

else{
    if((x_p-x[n-2]) > (x[n+1]-x_p)){
        x3 = x[n+1];
        y3 = y[n+1];
    }
    else{
        x3 = x[n-2];
        y3 = y[n-2];
    }
}
a2 = (y3-y1)/(x3-x1)/(x3-x2)-(y2-y1)/(x2-x1)/(x3-x2);
a1 = (y2-y1)/(x2-x1)-a2*(x1+x2);
a0 = y1-a1*x1-a2*x1*x1;

res=a0+a1*x_p+a2*x_p*x_p;

return res;
}