Overview

For each part-time job, calculate “hourly wage \(\times\) hours worked” and sum them all up to obtain the total salary for this month.

Analysis

The amount received from the \(i\)-th part-time job, given an hourly wage of \(A_i\) yen and working hours of \(T_i\) hours, is

\(A_i \times T_i\)

yen.
Therefore, the desired answer is

\(A_1T_1 + A_2T_2 + \cdots + A_NT_N\)

For example,

• Job 1: hourly wage \(1000\) yen, \(3\) hours → \(3000\) yen
• Job 2: hourly wage \(1200\) yen, \(2\) hours → \(2400\) yen

Then the total is \(3000 + 2400 = 5400\) yen.

Naive Approach

One might consider “adding the hourly wage for each individual hour worked.”
However, since \(T_i\) can be up to \(10^6\) and the number of job types \(N\) can be up to \(10^5\), processing hour by hour would be extremely wasteful.

In practice, it suffices to compute \(A_i \times T_i\) all at once for each job, so we only need to look at each job once.

Algorithm

1. Read \(N\).
2. Initialize a variable total representing the total amount to \(0\).
3. For each part-time job, read the hourly wage \(A_i\) and working hours \(T_i\).
4. Perform total += A_i * T_i.
5. Finally, output total.

Complexity

• Time complexity: \(O(N)\)
• Space complexity: \(O(1)\)

Implementation Notes

• You can read each line and immediately add a * t to the total; there is no need to store values in an array.

• The total amount can be as large as approximately \(10^5 \times 10^6 \times 10^6 = 10^{17}\). Python’s integer type can handle this without any issues.

• The provided code uses sys.stdin.buffer.readline for fast input, but the essence of this problem is simply “summing up products.”

  Source Code

import sys

def main():
    input = sys.stdin.buffer.readline
    N = int(input())
    total = 0
    for _ in range(N):
        a, t = map(int, input().split())
        total += a * t
    print(total)

if __name__ == "__main__":
    main()

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This editorial was generated by gpt-5.4-high.