// yukicoder: No.633 バスの運賃
// 2019.5.1 bal4u

#include <stdio.h>

#if 1
#define gc() getchar_unlocked()
#define pc(c) putchar_unlocked(c)
#else
#define gc() getchar()
#define pc(c) putchar(c)
#endif
int in()    // 非負整数の入力
{
	int n = 0, c = gc();
	do n = 10 * n + (c & 0xf), c = gc(); while (c >= '0');
	return n;
}

int a[102]; int n;

int main()
{
	int i, b, c, s, ans;
	
	n = in();
	for (i = 1; i < n; i++) a[i-1] = in();
	s = 0, ans = 0; for (i = 0; i < n; i++) {
		b = in(), c = in();
		s += c - b;
		ans += s*a[i];
	}
	printf("%d\n", ans);
	return 0;
}