n = int(input())
a = [int(input()) for _ in range(n-1)]
acc = [0]

for ai in a:
    acc.append(acc[-1]+ai)
    
ans = 0
l = [0]*n

for i in range(n):
    bi, ci = map(int, input().split())
    
    for j in range(n):
        if l[j]-bi>=0:
            ans += bi*(acc[i]-acc[j])
            l[j] -= bi
            break
        else:
            ans += l[j]*(acc[i]-acc[j])
            l[j] = 0
            bi -= l[j]
    
    l[i] += ci
    
print(ans)