// yukicoder: No.881 sin(x)xの和
// bal4u 2019.9.7

#include <stdio.h>

//// 高速入力
#if 0
int getchar_unlocked(void);
#define gc() getchar_unlocked()
#else
#define gc() getchar()
#endif

int in() {   // 非負整数の入力
	int n = 0, c = gc();
	do n = 10 * n + (c & 0xf); while ((c = gc()) >= '0');
	return n;
}

double inF() {   // 実数の入力
	int minus = 0;
	double x, y;
	int n = 0, c = gc();
	if (c == '-') minus = 1, c = gc();
	do n = 10*n + (c & 0xf), c = gc(); while (c >= '0');

	if (c == '.') {
		x = 0;
		y = 1, c = gc();
		do y *= 0.1, x += y * (c & 0xf), c = gc(); while (c >= '0');
		x += n;
	} else x = n;
	if (minus) x = -x;
	return x;
}

#define PI 3.1415926535897932384626433832795
int main()
{
	int n; double s, x, a;
	
	n = in();
	s = 0; while (n--) {
		scanf("%lf%lf", &x, &a);
//		x = inF(), a = inF();
		s += a;
	}
	printf("%.12lf\n", s * PI);
	return 0;
}