未处理的“System.FormatException”类型的异常出现在 mscorlib.dll 中 是怎么回事？

这是一个很简单的控制台程序，看不出来语法上有什么问题，但是一编译调试就会出现异常：未处理的“System.FormatException”类型的异常出现在 mscorlib.dll 中， 其他信息：Input string was not in a correct format.
请问该怎么解决？


// 纪录30个考生5年内每年15次的考试成绩，并打印各次考试的平均成绩各个学生每学年平均成绩

using System;

class score
{
public static void Main()
{
int[ , , ] myscore = new int [ 5, 15, 30];
int[ , ] scoreAveragePerYear = new int [ 5, 30];
int[ , ] scoreAveragePerTest = new int [ 5, 15];

Random rnd = new Random();
int abscore = 0;

for ( int year = 0; year <= 4; year++)
for ( int number = 0; number <= 14; number++)
for ( int student = 0; student <= 29; student++)
{
abscore = ( int) ( (rnd.Next() * 100) + 1);
myscore[ year, number, student] = abscore;
}

for ( int year = 0; year <= 4; year++)
for ( int number = 0; number <= 14; number=number+1)
{
scoreAveragePerTest[ year, number] = 0;
for ( int student = 0; student <= 29; student++)
scoreAveragePerTest[ year, number] += myscore[ year, number, student];
scoreAveragePerTest[ year, number] /= 30;
Console.WriteLine ( "The score average of the Year { 0}, Test { 1} is: { 2}", year + 1, number + 1, scoreAveragePerTest[ year, number]);
}
for ( int year = 0; year <= 4; year++)
for ( int student = 0; student <= 29; student++)
{
scoreAveragePerYear[ year, student] = 0;
for ( int number = 0; number <= 14; number++)
scoreAveragePerYear[ year, student] += myscore[ year, number, student];
scoreAveragePerYear[ year, student] /= 15;
Console.WriteLine ( "The score average of the Year { 0}, Student { 1} is: { 2}", year + 1, student + 1,scoreAveragePerYear[ year, student]);
}
}
}