代码：

#include <bits/stdc++.h>
using namespace std;
int main(){
	long long n = 0,m = 0,S[100001] = {0},b[100001] = {0};
	long long ai = 0, bi = 0;
	long long sum = 0;
	cin >> n >> m;
	for(int i = 0;i < n-1; i++)
	   cin >> S[i];
	for(int i = 0;i < m; i++)
	{
		cin >> ai>> bi;
		b[ai-1] += bi;
    }
	for(int i = 0;i < n-1; i++){
		if(b[i] != 0)
		{
			if(b[i] >= b[i+1])
			{
				b[i+1] = b[i+1] - b[i];
			}
			else
			{
				b[i+1] = b[i+1] - b[i];
			}
		}
	}
	if(b[n - 2] < 0)
	{
		cout << "Impossible" <<endl;
        return 0;
	}
	else
	{
		for(int i = 0;i< n - 1;i++)
		{
			sum += b[i] * S[i];
		}
		cout << sum << endl;
    }
	return 0;
}